The Manhattan Project

 General Leslie Groves, J. Robert Oppenheimer, and other scientists examining the site of the Trinity test

"The object of the project is to produce a practical military weapon in the form of a bomb...."

On July 13, 2011, U.S. Interior Secretary Kenneth Lee “Ken” Salazar said “The secret development of the atomic bomb in multiple locations across the United States was an important story and one of the most transformative events in our nation‟s history”.


He continued, “The Manhattan Project ushered in the atomic age, changed the role of the United States in the world community, and set the stage for the Cold War”.

The eight 'Signature Facilities' that were part of the Manhattan Project were:

The Metallurgical Laboratory (“Met Lab”), University of Chicago (Chemistry Building and Chicago Pile (CP-1) site)

X-10 Graphite Reactor, Oak Ridge, Tennessee

K-25 Gaseous Diffusion Process Building, Oak Ridge

Y-12 Beta-3 Racetracks, Oak Ridge

B Reactor, Hanford, Washington

Chemical Separations Building (T Plant), Hanford

V-Site Assembly Building, Los Alamos, New Mexico

Trinity Site, Alamogordo, New Mexico. 

Within each facility there were, what can be called now ‘Signature Artefacts', such as the Alpha and Beta calutron magnets and the small calutron in Y-12. Building 9204-3 (Beta 3), still has two Beta calutron racetracks (one still remains in standby today). These instruments or ‘tools' are now considered historical artefacts, but what I want to capture in these pages is a little bit about what actually happened during the 40’s.

As a starting point I have used here, more or less word-for-word, Chapters III and V of “Atomic Energy for Military Purposes” by H. D. Smyth. This report is subtitled “The Official Report on the Development of the Atomic Bomb” 1940-1945. Two very valuable additional reports were “The Manhattan Project” by Terrence R. Fehner and F. G. Gosling (2010), and “Manhattan: The Army and the Atomic Bomb” by Vincent C. Jones (1985). Additions extracted from other sources are in italics.

The Manhattan Engineering District

The Manhattan Engineering District (MED, also known as the Manhattan Project) was initially headquartered in New York City. It had no prescribed territorial limits and functioned as a special District (an U.S. Army term) for directing the atomic bomb project. Still today the U.S. Army Corps of Engineers uses ‘Division’ and 'District', with, for example, Southwestern Division in the U.S., and Middle East District and Europe District.

As such, the Manhattan Engineering District supervised research, development and testing, plant construction, and production programs relating to the project. The District also administered numerous laboratories and field installations, including the Clinton Engineer Works at Oak Ridge, Tenn., which performed separation of uranium isotopes; the Hanford Engineer Works at Richland, Wash., which produced plutonium; and the laboratory at Los Alamos, N. Mex., which performed final processing of the fissionable materials and assembled the finished atomic bombs. The District also maintained facilities in more than 30 cities and, during its peak operations, employed more than 129,000 persons.

By the end of 1945, the overall cost of the Manhattan Project had reached nearly $2 billion. And it had required the construction and use of 37 installations in 19 different U.S. states and Canada. 

According to one estimate, the Manhattan Project cost $2.2 billion from 1942 to 1946 ($22 billion in 2008 dollars), which is much greater than the original cost and time estimate of approximately $148 million for 1942 to 1944. General Leslie Richard Groves Jr., who managed the Manhattan Project, has written that Members of Congress who inquired about the project were discouraged by the Secretary of War from asking questions or visiting sites. After the project was under way for over a year, in Feb. 1944, War Department officials received essentially a ‘blank check’ for the project from Congressional leadership who “remained completely in the dark” about the Manhattan Project, according to Groves and other experts.

In 2008 dollars, the cumulative cost of the Manhattan project over 5 fiscal years was approximately $22 billion, as compared to approximately $98 billion for the Apollo program over 14 fiscal years. A measure of the U.S. commitment to the programs was their relative shares of the federal outlays during the years of peak funding: for the Manhattan Project, the peak year funding was 1% of federal outlays, and for the Apollo program, 2.2%. Another measure of the commitment was their relative shares of the nation’s gross domestic product (GDP) during the peak years of funding: for both the Manhattan Project and the Apollo program, the peak year funding reached 0.4% of GDP.

General Leslie Richard Groves Jr. (1896-1970), oversaw the construction of the Pentagon and directed the Manhattan Engineering District (1942-1947). He later went on to be vice-president of Sperry Rand

On July 16, 1945, the first atomic bomb was successfully detonated at Alamogordo, N. Mex. (more usually called the Trinity Site).

On Aug. 6, 1945, and on Aug. 9, 1945, atomic bombs were dropped on Hiroshima and Nagasaki, Japan, respectively.

During the summer of 1944, the problem of postwar control of nuclear energy began to receive serious consideration. In a memorandum of July 27, 1944, James Bryant Conant advocated the formation of a "Commission on Atomic Energy" to be charged with postwar development of nuclear energy for civilian and military purposes. Over the next few months, Conant, along with Vannevar Bush, urged the establishment of such a Commission.

James Bryant Conant (1983-1978), was a chemist, President of Harvard University, and first U.S. Ambassador to Wests Germany. He worked on poison gases for the U.S. Army in WW I, in particular lewisite. In 1941 he became chairman of the U.S. National Defense Research Committee (NDRC). 

Vannevar Bush (1890-1974), was an engineer, and during WW II headed the U.S. Office of Scientific Research and Development. He founded Raytheon, was chairman of Merck, regent of the Smithsonian Institution, and 'invented' memex, a proto-hypertext system. He was instrumental in creating the U.S. National Science Foundation

On May 4, 1945, Henry Lewis Stimson appointed an Interim Committee "to survey and make recommendations on postwar research, development, and controls, as well as legislation necessary to effectuate them". The committee was chaired by the Secretary of War and also included the U.S. Secretary of State James Francis Byrnes, Assistant Secretary of State William Lockhart Clayton, Undersecretary of the Navy Ralph Austin Bard, OSRD's Karl Taylor Compton, Bush, and Conant. The work of the Interim Committee eventually resulted in the establishment of the U.S. Atomic Energy Commission (AEC) by the Atomic Energy Act of Aug. 1, 1946. All phases of nuclear energy research and production came under the control of the AEC on Jan. 1, 1947.

Henry Lewis Stimson (1867-1950) was the U.S. Secretary of War for both WW I and WW II. 

The NDRC was assimilated into the U.S. Office of Scientific Research and Development (OSRD) in 1941. 

On December 31, 1946, Truman signed the executive order that transferred peacetime control of the program to the AEC, marked the end of the Manhattan Project, and transferring ownership of 37 military installations to the civilian commission. The Army also provided the new supervisors of the nation’s nuclear program with a cadre of nearly 2,000 U.S. troops and 4,000 U.S. government civilian employees to provide continuity to the program, just as Groves had called for a year earlier.

The Manhattan Engineer District continued about 6 months longer, solely as an administrative agency to close out the project and handle personnel assigned to the AEC during the transition period. On Dec. 31, 1946, a joint Army-Navy organization, the U.S. Armed Forces Special Weapons Project (AFSWP), was established to assume the military functions of the Manhattan Engineering District.

During WW I, Thomas Edison suggested "to the Navy that it should bring into the war effort at least one physicist in case it became necessary to calculate something”. The Manhattan project brought together 1,000’s of physicists, chemists, mathematicians, and engineers. After WW II the effect of science on our society has been incredible, physicists have now become “experts” on almost anything, and today every government understands that science is a major element in asserting its national power.

How it all started

The announcement of the hypothesis of fission and its experimental confirmation took place in Jan. 1939. There was immediate interest in the possible military use of the large amounts of energy released in fission. Beginning in 1939, some key scientists expressed concern that Germany might be building an atomic weapon. The early efforts both at restricting publication and at getting government support were stimulated largely by a small group of foreign-born physicists centered on Leo Szilárd and including Eugene Paul Wigner, Edward Teller, Victor Frederick Weisskopf, and Enrico Fermi. At the same time it was proposed that the United States accelerate atomic research in response to the perceived German threat.

The ‘Martians’, four brilliant scientists who were born in the same neighbourhood in Budapest, Hungary, allegedly earned their nickname from Enrico Fermi. A refugee from Italy himself, Fermi quipped that there must have been a spaceship from Mars that landed in Budapest, dropping off the extraordinarily gifted Edward Teller, Eugene Wigner, John von Neumann, and Leo Szilárd.

In fact, one of the remarkable things about the Manhattan Project was that so many “one in a generation” minds gathered at Los Alamos to collaborate on designing and building the bomb. The Manhattan Project boasted eight Nobel Prize laureates: Enrico Fermi, James Chadwick, Niels Henrik David Bohr, Arthur Holly Compton, Ernest Orlando Lawrence, James Franck, Harold Clayton Urey, and Isidor Isaac Rabi. After the war, over a dozen Manhattan Project veterans would go on to win Nobel Prizes, including Hans Albrecht Bethe, Emilio Gino Segré, Eugene Paul Wigner, Richard Phillips Feynman, and Glenn Theodore Seaborg. Then there were brilliant scientists, including Julius Robert Oppenheimer, Chien-Shiung Wu, and John von Neumann, whose valuable contributions were essential to the success of the project and changed science and mathematics forever.

Julius Robert Oppenheimer (1904-1967) was wartime head of Los Alamos, and is often called the “farther of the atomic bomb”. His achievements include the Born–Oppenheimer approximation for molecular wavefunctions, work on the theory of electrons and positrons, the Oppenheimer–Phillips process in nuclear fusion, the Tolman-Oppenhiemer-Volkoff limit for the mass of stars, and the first prediction of quantum tunneling. After WW II, he became director of the Institute for Advanced Study in Princeton.

In the spring of 1939 the group of Szilárd enlisted Niels Bohr's cooperation in an attempt to stop publication of further data by voluntary agreement, but publication continued freely for about another year although a few papers were withheld voluntarily by their authors. At the April 1940 meeting of the Division of Physical Sciences of the U.S. National Research Council, Gregory Breit proposed forming a censorship committee to control publication in all American scientific journals. This arrangement was very successful in preventing publication and was still nominally in effect, in modified form, through to June 1945.

We have to remember that the ‘modern’ science community of that time was a web of connections, both as friends and professional. For example, Oppenheimer worked with Bohr, who was teaching Teller, and fellow Hungarian Szilárd met Wigner in 1921 while studying under Einstein, von Laue, and Planck in Berlin. When in Jan. 1939 at a conference on low temperature physics, Niels Bohr delivered a lecture on splitting the nucleus of uranium, he aroused the concern of numerous conscientious scientists. Leo Szilárd, "obviously concerned, took [Edward Teller] aside", and said, "Let's be careful. Let's not talk about this too much". Teller agreed and "concentrated on returning the conference to the subject of low temperatures". Szilárd, Teller and others were aware that, given the capabilities of German physics and the inclinations of Hitler, the world might be endangered by this scientific discovery.

Strictly speaking Bohr did not deliver a lecture on splitting the nucleus of uranium, what he made was a short unannounced intervention (interruption) at a conference on theoretical physics. Attendees were amazed by the announcement. He revealed that Otto Hahn and Friedrich Wilhelm “Fritz” Strassmann of the Kaiser Wilhelm Institute in Germany, and Lise Meitner with Otto Robert Frisch, both Austrian physicists in exile (Meitner in Sweden and Frisch in Denmark), had been successful in splitting the atom. Among the people sitting in that room were Fermi, Urey, Rabi, Teller, Bethe, Roberts, and Johnson, all names we will encounter in these pages on the Manhattan Project.  

The first contact with the U.S. government was made by George Brexton Pegram of Columbia in March 1939. Pegram telephoned to the Navy Department and arranged for a conference between representatives of the Navy Department and Fermi. The only outcome of this conference was that the Navy expressed interest and asked to be kept informed. 

George Brexton Pegram (1876-1958) brokered a meeting between Fermi and the U.S. Navy on the prospects of the atomic bomb. He was chair of the physics department in Columbia University and help found Brookhaven National Laboratory. Fermi, Rabi, Szilárd Anderson, and Zinn all worked in the Columbia physics department. 

The next attempt to interest the government was stimulated by Szilárd and Wigner. In July 1939 they conferred with Einstein, and a little later Einstein, Wigner, and Szilárd discussed the problem with Alexander Sachs of New York. In the fall Sachs, supported by a letter from Einstein, explained to President Roosevelt the desirability of encouraging work in this field.

Alexander Sachs (1893-1973) was Lehman Brothers chief economist, and friend of Roosevelt (and some called him also a pompous financier). 

On Oct. 11, 1939, Sachs met with the President to discuss the letter written by Albert Einstein the previous August. Einstein had written to inform Roosevelt that recent research on chain reactions utilizing uranium made it probable that large amounts of power could be produced by a chain reaction and that, by harnessing this power, the construction of “extremely powerful bombs...” was conceivable. Einstein believed the German government was actively supporting research in this area and urged the United States government to do likewise. Sachs read from a cover letter he had prepared and briefed Roosevelt on the main points contained in Einstein’s letter. Initially the President was noncommittal and expressed concern over locating the necessary funds, but at a second meeting over breakfast the next morning Roosevelt became convinced of the value of exploring atomic energy.

Einstein drafted his famous letter with the help of the Hungarian emigre physicist Leo Szilárd, one of a number of European scientists who had fled to the United States in the 30’s to escape Nazi and Fascist repression. Szilárd was among the most vocal of those advocating a program to develop bombs based on recent findings in nuclear physics and chemistry. Those like Szilárd and fellow Hungarian refugee physicists Edward Teller and Eugene Wigner regarded it as their responsibility to alert Americans to the possibility that German scientists might win the race to build an atomic bomb and to warn that Hitler would be more than willing to resort to such a weapon. 

But Roosevelt, preoccupied with events in Europe, took over two months to meet with Sachs after receiving Einstein’s letter. Szilárd and his colleagues interpreted Roosevelt’s inaction as unwelcome evidence that the President did not take the threat of nuclear warfare seriously.

Roosevelt wrote back to Einstein on Oct. 19, 1939, informing the physicist that he had setup a committee consisting of Sachs and representatives from the Army and Navy to study uranium. Events proved that the President was a man of considerable action once he had chosen a direction. In fact, Roosevelt’s approval of uranium research in October 1939, based on his belief that the United States could not take the risk of allowing Hitler to achieve unilateral possession of “extremely powerful bombs” was merely the first decision among many that ultimately led to the establishment of the only atomic bomb effort that succeeded in WW II - the Manhattan Project.

The Uranium Committee

The President appointed a committee, known as the "Advisory Committee on Uranium", consisting of Lyman James Briggs (director of the U.S. National Bureau of Standards) as chairman, Colonel K. F. Adamson of the Army Ordnance Department, and Commander G. C. Hoover of the Navy Bureau of Ordnance, and requested this committee to look into the problem. This was the only committee on uranium that had official status up to the time of the organisation of the National Defence Research Committee in June 1940. The committee met very informally and included various additional scientific representatives in its meetings.

The first meeting of the Uranium Committee was on Oct. 21, 1939 and included, besides the committee members, F. L. Mohler, Sachs, Szilárd, Wigner, Teller, and Richard Brooke Roberts. The result of this meeting was a report dated Nov. 1, 1939, and transmitted to President Roosevelt by Briggs, Adamson, and Hoover. This report made eight recommendations, and specifically mentioned both atomic power and an atomic bomb as possibilities. It specifically recommended procurement of 4 tons of graphite and 50 tons of uranium oxide for measurements of the absorption cross section of carbon.

The first transfer of funds ($6,000) from the Army and Navy to purchase materials in accordance with the recommendation of Nov. 1, 1939, is reported in a memorandum from Briggs to General Edwin Martin Watson (President Roosevelt's aide) on Feb. 20, 1940. The next meeting of the "Advisory Committee on Uranium" was on April 28, 1940 and was attended by Sachs, Wigner, Pegram, Fermi, Szilárd, Briggs, Admiral Harold Gardiner Bowen, Colonel Adamson, and Commander Hoover. 

The Army and Navy each gave $3,000 to National Bureau of Standards, which gave the money to Columbia University (i.e. Fermi), who used it to buy an ‘inordinate’ quantity of graphite. The Army was only convinced to allocate these fund when it learned that the President “was interested in this project”. 

$6,000 today does not sound like very much, but at the time (February 1940) it reflected the importance attached to the Fermi-Szilárd pile experiments already underway at Columbia University. Building upon the work performed in 1934 demonstrating the value of moderators in producing slow neutrons, Fermi thought that a mixture of the right moderator and natural uranium could produce a self-sustaining chain reaction. Fermi and Szilárd increasingly focused their attention on carbon in the form of graphite. Perhaps graphite could slow down, or moderate, the neutrons coming from the fission reaction, increasing the probability of causing additional fissions in sustaining the chain reaction. A pile containing a large amount of natural uranium could then produce enough secondary neutrons to keep a reaction going.

There was, however, a large theoretical gap between building a self-generating pile and building a bomb. Although the pile envisioned by Fermi and Szilárd could produce large amounts of power and might have military applications (powering naval vessels, for instance), it would be too big for a bomb. It would take separation of uranium-235 or substantial enrichment of natural uranium with uranium-235 to create a fast-neutron reaction on a small enough scale to build a usable bomb. While certain of the chances of success in his graphite power pile, Fermi, in 1939, thought that there was “little likelihood of an atomic bomb, little proof that we were not pursuing a chimera”. 

By the time of this meeting two important new factors had come to light. First, it had been discovered that the uranium fission caused by neutrons of thermal velocities occurred in the uranium-235 isotope only. Second, it had been reported that a large section of the Kaiser Wilhelm Institute in Berlin had been set aside for research on uranium. 

Fission may take place in any of the heavy nuclei after capture of a neutron. However, low-energy (slow, or thermal) neutrons are able to cause fission only in those isotopes of uranium and plutonium whose nuclei contain odd numbers of neutrons (e.g. U-233, U-235, and Pu-239). The fission cross-section increases dramatically (when competing with the capture cross-section) for thermal neutron (<0.025 eV) on U-235 than for higher energies (1/v dependence).

Within the next few weeks a number of people, particularly Sachs, urged the importance of greater support and of better organisation. Their hand was strengthened by the Columbia results (as reported, for example, in a letter from Sachs to General Watson on May 15, 1940) showing that the carbon absorption was appreciably lower than had been previously thought and that the probability of carbon being satisfactory as a moderator was therefore considerable. Sachs was also active in looking into the question of ore supply. On June 1, 1940, Sachs, Briggs, and Urey met with Admiral Bowen to discuss approaching officials of the Union Miniere of the Belgian Congo. Such an approach was made shortly afterwards by Sachs.

In fact Edgar Sengier, head of Union Miniere, had already ordered a shipment of 1,200 tons of high-grade ore from the Shinkolobwe stockpile in the Congo, via Portuguese West Africa to New York. Oddly enough at the time the U.S. government was not interesting in this stockpile, but it was ‘discovered' when Standard Oil, working on the centrifuge process, opened negotiations. Thus, by an odd series of events, the U.S. found enough uranium ore, just when they needed it.  

The general status of the problem was discussed by a special advisory group called together by Briggs at the National Bureau of Standards on June 15, 1940. This meeting was attended by Briggs, Urey, Merle Anthony Tuve, Wigner, Breit, Fermi, Szilárd, and Pegram. After full discussion, the recommendation of the group to the Uranium Committee was that funds should be sought to support research on the uranium-carbon experiment along two lines:

further measurements of the nuclear constants involved in the proposed type of reaction,

experiments with amounts of uranium and carbon equal to about one fifth to one quarter of the amount that could be estimated as the minimum in which a chain reaction would sustain itself.

"It was estimated that about $40,000 would be necessary for further measurements of the fundamental constants and that approximately $100,000 worth of metallic uranium and pure graphite would be needed for the intermediate experiment".

(Quotations from memorandum of Pegram to Briggs, dated Aug. 14, 1940)

National Defence Research Committee

Before any decisions made at this meeting could be put into effect, the organisation of the National Defence Research Committee (NDRC) was announced in June 1, 1940, and President Roosevelt gave instructions that the Uranium Committee should be reconstituted as a subcommittee of the NDRC, reporting to Vannevar Bush (chairman, NDRC). The membership of this reconstituted Uranium Committee was as follows: Briggs (Chairman), Pegram, Urey, Beams, Tuve, Ross Gunn and Breit. On authorisation from Briggs, Breit consulted Wigner and Teller frequently although they were not members of the committee.

The National Defence Research Committee, with Bush at its head, reorganised the Uranium Committee into a scientific body and eliminated military membership. Not dependent on the military for funds, as the Uranium Committee had been, the National Defence Research Committee would have more influence and more direct access to money for nuclear research. In the interest of security, Bush barred foreign-born scientists from committee membership and blocked the further publication of articles on uranium research. Retaining program responsibilities for uranium research in the new organisation as setup (among the National Defence Research Committee’s early priorities were studies also on radar, proximity fuzes, and anti-submarine warfare), the Uranium Committee recommended that isotope separation methods and the chain reaction work continue to receive funding for the remainder of 1940. Bush approved the plan and allocated the funds.

Through to Dec. 1941, a number of contracts were let. Their number and total amount grew gradually. Urey began to work on isotope separation by the centrifuge method under a Navy contract in the fall of 1940. Other contracts were granted to Columbia University, Princeton University, Standard Oil Development Company, Cornell University, Carnegie Institution of Washington, University of Minnesota, Iowa State College, John Hopkins University, National Bureau of Standards, University of Virginia, University of Chicago, and University of California in the course of the winter and spring of 1940-1941, until by Nov. 1941 the total number of projects approved was sixteen, totalling about $300,000.

The Uranium Committee as formed in the summer of 1940 continued substantially unchanged until the summer of 1941. At that time the main committee was somewhat enlarged and subcommittees formed on isotope separation (Urey), theoretical aspects (Fermi, Sziland), power production (Fermi, Szilárd) and heavy water (Urey). It was thereafter called the Uranium Section or the S-l Section of NDRC.

It is said that using S-1 and dropping the word uranium was, at least in part, for security reasons.

We have to remember that Germany invaded Norway in early April, 1940, securing control of the Norsk Hydro plant, the only large facility in the world producing heavy water (deuterium oxide). This plant was sabotaged by the Norwegians to prevent the Germans acquiring the heave water. Harold Urey discovered the isotope deuterium in 1931 and was later able to concentrate it in water. Heavy water is an alternative neutron moderator to graphite.  Because they do not require uranium enrichment, heavy water reactors are more of a concern for nuclear proliferation. The breeding and extraction of plutonium can be a relatively rapid and cheap route to building a nuclear weapon, as chemical separation of plutonium from fuel is easier than isotopic separation of uranium-235 from natural uranium. 

In the spring of 1941, Briggs, feeling that an impartial review of the problem was desirable, requested Bush to appoint a review committee. Bush then formally requested Frank B. Jewett, president of the National Academy of Sciences, to appoint such a committee. Jewett complied, appointing Compton (chairman), William David Coolidge, Lawrence, John Clarke Slater, John Hasbrouck Van Vleck, and Bancroft Gherardi Jr. This committee was instructed to evaluate the military importance of the uranium problem and to recommend the level of expenditure at which the problem should be investigated.

Arthur Holly Compton (1892-1962) won the Nobel Prize in Physics in 1927 for the discovery of the Compton effect, thus demonstrating the particle nature of electromagnetic radiation. he was leader of the Metallurgical Laboratory in Chicago, and later was in charge of plutonium production for the bomb.

Ernest Orlando Lawrence (1901-1958) won the Nobel Prize in Physics in 1939 for the invention of the cyclotron, and was responsible for isotope separation and uranium enrichment with the calutrons.   

Pegram's summarised progress in a report dated Feb. 15, 1941 showing that the main progress had been made on understanding the slowing down of neutrons in graphite, on the number of neutrons emitted in fission, and on the design and size of lattices.

The slowing down of neutrons in graphite was investigated by studying the intensity of activation of various detectors (rhodium, indium, iodine) placed at various positions inside a rectangular graphite column when a source of neutrons was placed inside. Using cadmium screens (cadmium having a very large absorption cross-section for thermal neutrons) the effects of resonance and thermal neutrons were investigated separately. These results, coupled with theoretical studies of the diffusion of thermal neutrons, laid a basis for future calculations of the number of thermal and resonance neutrons to be found at any point in a graphite mass of given shape when a given neutron source is placed at a specified position within or near the graphite. The experiments on slowing down neutrons showed that high-energy neutrons such as those from fission were practically all reduced to thermal energies after passing through 40 cm or more of graphite. A piece of uranium placed in a region where thermal neutrons were present absorbed the thermal neutrons and as fission occurs re-emits fast neutrons, which were easily distinguished from the thermal neutrons. By a series of measurements with and without uranium present and with various detectors and absorbers, it was possible to get a value for the constant η, the number of neutrons emitted per thermal neutron absorbed by uranium. This is not the number of neutrons emitted per fission, but is somewhat smaller than that number since not every absorption causes fission.

Chicago Pile-1 (CP-1), the site of the first human-made, self-sustaining nuclear reaction. It consisted of a large, monolithic pile of uranium pellets and graphite blocks, with cadmium, indium, and silver control rods, but no radiation shield and cooling system. This is in fact just one of the multiple builds and re-builds of the same pile.

The fundamental idea of a ‘pile’ was a lattice structure with uranium concentrated in lumps regularly distributed in a matrix of (graphite) moderator. If the uranium and the moderator are mixed homogeneously, the neutrons on average will lose energy in small steps between passages through the uranium. So in the course of their reduction to thermal velocity the chance of their passing through uranium at any given velocity, e.g. at a velocity corresponding to resonance absorption, is great. But, if the uranium is in large lumps spaced at large intervals in the moderator, the amounts of energy lost by neutrons between passages from one lump of uranium to another will be large and the chance of reaching a uranium lump with energy just equal to the energy of resonance absorption is relatively small. Thus the chance of absorption by uranium-238 to produce uranium-239, compared to the chance of absorption as thermal neutrons to cause fission, may be reduced sufficiently to allow a chain reaction to take place. If one knew the exact values of the cross sections of each uranium isotope for each type of absorption and every range of neutron speed, and had similar knowledge for the moderator, one could calculate the ‘optimum lattice’, i.e. the best size, shape and spacing for the lumps of uranium in the matrix of moderator. Since such data were only partially known, experiments were initiated at Columbia, and continued at Princeton in Feb. 1941. The absorption of neutrons by uranium was measured under conditions similar to those expected in a chain-reacting pile employing graphite as moderator.

During 1939 and 1940 most of the work done on isotope separation and the chain reaction pile was performed in university laboratories by academic scientists funded primarily by private foundations. While the U.S. federal government began supporting uranium research in 1940, the pace appeared too leisurely to the scientific community and failed to convince scientists that their work was of high priority. Certainly few were more inclined to this view than Ernest O. Lawrence, director of the Radiation Laboratory at the University of California

in Berkeley. Lawrence was among those who thought that it was merely a matter of time before the United States was drawn into WW II, and he wanted the government to mobilise its scientific forces as rapidly as possible.

Specifically what Lawrence had on his mind in early 1941 were experiments taking place in his own laboratory using samples produced in the cyclotron. Studies on uranium fission fragments by Edwin Mattison McMillan and Philip Hauge Abelson led to the chemical identification of element 93, neptunium, while research by Glenn Theodore Seaborg revealed that an isotope of neptunium decayed to yet another transuranium (man-made) element. In February, Seaborg identified this as element 94, which he later named plutonium. By May 1941, he had proven that plutonium-239 was 1.7 times as likely as uranium-235 to fission. This finding made the Fermi-Szilárd experiment more important than ever as it suggested the possibility of producing large amounts of the fissionable plutonium in a uranium pile using plentiful uranium-238 and then separating the different elements chemically. Surely this would be less expensive and simpler than building isotope-separation plants. Lawrence, demonstrating his characteristic energy and impatience, launched a campaign to speed up uranium research. He began by proposing to convert his smaller cyclotron into a spectrograph to produce uranium-235. Since both the cyclotron and the spectrograph used a vacuum chamber and an electromagnet, this conversion was relatively uncomplicated. Lawrence then took his case to Karl T. Compton and Alfred Lee Loomis at Harvard University, both doing radar work for the National Defense Research Committee. Infected by Lawrence’s enthusiasm, Compton forwarded Lawrence’s optimistic assessment on uranium research to Bush, warning that Germany was undoubtedly making progress and that Briggs and the Uranium Committee were moving too slowly. Compton also noted that the British were ahead of their American colleagues, even though, in his opinion, they were inferior in both numbers and ability.

Chain of the transuranic elements. Neptunium (Np) was first synthesised by Edwin McMillan and Philip H. Abelson at the Berkeley Radiation Laboratory in 1940. Plutonium (Pu) was first produced and isolated in 1940 by Glenn T. Seaborg, Joseph W. Kennedy, Edwin M. McMillan, and Arthur C. Wahl. Americium (Am) was first produced in 1944 by the group of Glenn T. Seaborg from Berkeley, California, at the Metallurgical Laboratory of the University of Chicago, a part of the Manhattan Project. Curium (Cm) was first intentionally produced and identified in July 1944 by the group of Glenn T. Seaborg at the University of California, Berkeley. Other elements, such as Californium (Cf), were produced and identified after WW II.

Some books mention plutonium as being man-made, or discovered, or synthesised, but few mention plutonium-244, the only naturally occurring plutonium isotope, with a half-life of 80.8 million years. It has been postulated that this isotope has existed since the creation of Earth about 4.5 billion years ago (all heavy elements were formed in supernova explosions). It should also be mentioned that uranium in the Earth’s crust is also producing naturally plutonium, albeit on the order of one part in 1011 in pitchblende. And it is also worthwhile mentioning that apart from its formation in today's nuclear reactors, plutonium was formed by the operation of naturally-occurring nuclear reactors in uranium deposits at Oklo in what is now Gabon, some two billion years ago. However, most of the plutonium in the Earth’s environment comes from weapons tests, which released about 3 tons of plutonium into the atmosphere. There exists also plutonium-245, plutonium-246, and plutonium-247, all with half-lives of a few days. In fact, plutonium has 23 known isotopes, ranging in mass from 228 to 247, and 9 isotopes have metastable states that usually last less than 1 second.

Americium-241, with a half-life of 432 years, was the first americium isotope to be isolated, and is still the one used today in many domestic smoke detectors, although they have been banned in some countries and are being replaced by smoke detectors using the photoelectric circuit.

In mid-2014 a plan was announced to extract americium-241 from the UK plutonium stockpile, much of it old. According to the U.K. National Nuclear Laboratory (NNL), about 250 kg of old civil plutonium (originally with about 10-14% plutonium-241) will yield 10 kg of americium-241. The European Space Agency is paying NNL to produce americium-241 for 10-watt (e) radioisotope thermoelectric generators (RTG’s) using very pure americium-241 recovered from old civil plutonium, as the isotope is much less expensive than plutonium-238 (now scarce). NASA has now decided to pay for the cost of domestic plutonium-238 processing, but the U.S. still has about 35 kg of plutonium-238 in reserve. Americium costs about $1500 per gram, whereas the U.S. stopped producing plutonium-238 in 1988 (and bought some 16.5 kg produced at Mayak) and the Russians have also stopped producing and selling it internationally. My understanding is that the Mars rover Curiosity was fueled with 4.8 kg of plutonium-238, to produce 2 kW (th), the equivalent would have been around 15-20 kg of americium-241. Around 250 plutonium-powered pacemakers were manufactured, and 22 of them are still in service after 25 years (there were discontinued when it was found that they would not remain intact after cremation). 

Ernest O. Lawrence bombarding uranium with neutrons discovered that the capture of fast neutrons by uranium-238 transmuted that isotope first into element 93 and then into element 94, which they named neptunium and plutonium, respectively. After further investigation of these transuranium elements, neither of which was then known to exist in nature, Lawrence's group concluded that plutonium had the same fission characteristics as uranium-235, i.e. it could be split by neutrons and would, in turn, release more neutrons. Uranium-238, hitherto regarded as worthless for energy purposes, was in fact a prime source. Furthermore, as there was reason to believe that chemical separation of plutonium from uranium might prove more practicable than isotopic separation of uranium-235 from uranium-238, chances that an atomic bomb based on a fast neutron chain reaction could be built were tremendously increased.

In internal communications uranium-235 was often referred to as “25”, and plutonium as “49”, for the element no. 94, isotope atomic number 239

Th review committee requested by Bush met in May, 1941 and submitted a report. On the basis of this report and the oral exposition by Briggs before a meeting of the NDRC, an appropriation of $267,000 was approved by the NDRC at its meeting of July 18, 1941, and the probability that much larger expenditures would be necessary was indicated. Bush asked for a second report with emphasis on engineering aspects, and in order to meet this request Oliver Ellsworth Buckley of the Bell Telephone Laboratories and L. W. Chubb of the Westinghouse Electrical and Manufacturing Company were added to the committee. The second report was submitted by Coolidge. As a result of new measurements of the fission cross section of uranium-235 and with increasing conviction that isotope separation was possible, in Sept. 1941, Compton and Lawrence suggested to J. B. Conant of NDRC, who was working closely with Bush, that a third report was desirable. Since Bush and Conant had learned during the summer of 1941 that the British also felt increasingly optimistic, the committee was asked to make another study of the whole subject. For this purpose the committee was enlarged by the addition of Warren Kendall Lewis, Robert Sanderson Mulliken, and George Bogdanovich Kistiakowsky. This third report was submitted by Compton on Nov. 6, 1941.

George Bogdanovich Kistiakowsky (1900-1982) developed the explosives RDX and HNX, was responsible for developing shaped charges and explosive lenses used in the Manhattan Project weapons, and later served as Eisenhower’s scientific advisor.  


The report of May 1941 mentioned first radioactive poisons, then atomic power, and finally atomic bombs, but the emphasis was on power generation. The report of Nov. 1941 was specifically concerned with the “possibility of an explosive fission reaction with uranium-235”. The Nov. report said "since our last report, the progress toward separation of the isotopes of uranium has been such as to make urgent a consideration of (1) the probability of success in the attempt to produce a fission bomb, (2) the destructive effect to be expected from such a bomb, (3) the anticipated time before its development can be completed and production be underway, and (4) the preliminary estimate of the costs involved".

Bush and Lawrence met in New York City. Though he continued to support the Uranium Committee, Bush recognized that Lawrence’s assessment was not far off the mark. Bush shrewdly decided to appoint Lawrence as an advisor to Briggs, a move that quickly resulted in funding for plutonium work at Berkeley and for the mass spectrograph of Alfred Otto Carl Nier at Minnesota. Bush also asked the National Academy of Sciences to review the uranium research program. Headed by Arthur Compton of the University of Chicago and including Lawrence, this committee submitted its unanimous findings on May 17, 1941. Compton’s committee, however, failed to provide the practical-minded Bush with the evidence he needed that uranium research would pay off in the event the United States went to war in the near future. Compton’s group thought that increased uranium funding could produce radioactive material that could be dropped on an enemy by 1943, a pile that could power naval vessels in three or four years, and a bomb of enormous power at an indeterminate point, but certainly not before 1945. Compton’s report discussed bomb production only in connection with slow neutrons, a clear indication that much more scientific work remained to be done before an explosive device could be detonated.

Beginning in 1940 there was some interchange of information with the British and during the summer of 1941 Bush learned that they had been reviewing the whole subject in the period from April to July, 1941. They too had been interested in the possibility of using plutonium; in fact, a suggestion as to the advisability of investigating plutonium was contained in a letter from J. D. Cockcroft to R. H. Fowler dated Dec. 28, 1940. Fowler, who was at that time acting as British scientific liaison officer in Washington, passed Cockcroft's letter on to Lawrence. The British never pursued the plutonium possibility, since they felt their limited manpower should concentrate on uranium-235. Chadwick, at least, was convinced that a uranium-235 bomb of great destructive power could be made, and the whole British group felt that the separation of uranium-235 by diffusion was probably feasible.

Accounts of the British opinion, including the first draft of the British report reviewing the subject, were made available to Bush and Conant informally during the summer of 1941, although the official British report of July 15, 1941, was first transmitted to Conant by G. P. Thomson on Oct. 3, 1941. Since, however, the British review was not made available to the committee of the National Academy of Sciences, the reports by the Academy committee and the British reports constituted independent evaluations of the prospects of producing atomic bombs.

The British report, prepared by a group codenamed the MAUD Committee (Committee for Military Application Uranium Detonation) and set up in spring 1940 to study the possibility of developing a nuclear weapon, maintained that a sufficiently purified critical mass of uranium-235 could fission even with fast neutrons. The British project was called “Tube Alloys”. Building upon theoretical work on atomic bombs performed by refugee physicists Rudolf Ernst Peierls and Otto Robert Frisch in 1940 and 1941, the MAUD report estimated that a critical mass of ten kilograms would be large enough to produce an enormous explosion. A bomb this size could be loaded on existing aircraft and be ready in approximately two years. Optimistically they had concluded that a uranium bomb could be built with an explosive power of 1,800 tons of TNT.

Americans had been in touch with the MAUD Committee since fall 1940, but it was the July 1941 MAUD report that helped the American bomb effort turn the corner. Here were specific plans for producing a bomb, produced by a distinguished group of scientists with high credibility in the United States, not only with Bush and Conant but with the President. The MAUD report dismissed plutonium production, thermal diffusion, the electromagnetic method, and the centrifuge and called for gaseous diffusion of uranium-235 on a massive scale. The British believed that uranium research could lead to the production of a bomb in time to affect the outcome of the war. While the MAUD report provided encouragement to Americans advocating a more extensive uranium research program, it also served as a sobering reminder that fission had been discovered in Nazi Germany almost three years earlier and that since spring 1940 a large part of the Kaiser Wilhelm Institute in Berlin had been set aside for uranium research. 

For example, in Germany in April 1942, Baron Manfred von Ardenne already had completed an operational magnetic isotope separator in his laboratory in Berlin Lichterfelde, and his associate Friedrich Georg “Frtiz” Houtermans having correctly calculated the critical mass of uranium-235 the previous year. The MAUD Committee calculated the critical mass of uranium-235 to be 25 pounds. Robert Oppenheimer himself, before joining the Manhattan Project, had estimated critical mass at about 100 kilograms (220 pounds). His theoretical group at Berkeley quickly upped that by three times to 300 kilograms (660 pounds). As late as August 1943, when theorists at Los Alamos provided a critical mass estimate of 40 kilograms (88 pounds), the United States' target number for enriched uranium production was still unknown.

Bush and Conant immediately went to work. After strengthening the Uranium Committee, particularly with the addition of Fermi as head of theoretical studies and Urey as head of isotope separation and heavy water research (heavy water was highly regarded as another neutron moderator), Bush asked yet another reconstituted National Academy of Sciences committee to evaluate the uranium program. This time he gave Compton specific instructions to address technical questions of critical mass and destructive capability, partially to verify the MAUD results.

In Aug. 1943, the British were invited to join the Manhattan Project. This was the so-called Quebec Agreement signed by President Roosevelt and Winston Churchill. The British team included James Chadwick, Cyril Stanley Smith, Otto Frisch, Rudolf Peierls, and Emil Julius Klaus Fuchs (who later was convicted of spying for the Soviet Union).    

Besides the official and semi-official conferences, there were many less formal discussions held, one of these being stimulated by Marcus Laurence Elwin Oliphant of England during his visit to the U.S. in the summer of 1941. The general conclusion was that the program should be pushed, and this conclusion in various forms was communicated to Bush by a number of people.

In the fall of 1941 Urey and Pegram were sent to England to get first-hand information on what was being done there. This was the first time that any Americans had been to England specifically in connection with the uranium problem. The report prepared by Urey and Pegram confirmed and extended the information that had been received previously.

As a result of the reports prepared by the National Academy committee, by the British, and by Urey and Pegram, and of the general urging by a number of physicists, Bush, as Director of the Office of Scientific Research and Development (of which NDRG was a part), decided that the uranium work should be pushed more aggressively.

Before the National Academy issued its third report and before Pegram and Urey visited England, Bush had taken up the whole uranium question with President Roosevelt and Vice-President Henry Agard Wallace. He summarised for them the British views, which were on the whole optimistic, and pointed out the uncertainties of the predictions. The President agreed that it was desirable to broaden the program, to provide a different organisation, to provide funds from a special source, and to open a complete interchange of information with the British. It was agreed to confine discussions of general policy to the following group: The President, Vice-President, Secretary of War, Chief of Staff, Bush, and Conant. This group was often referred to as the Top Policy Group.

The British knew that several kilograms of heavy water a day were being produced in Norway, and that Germany had ordered considerable quantities of paraffin to be made using heavy hydrogen. It was difficult to imagine a use for these materials other than in work on the uranium problem. They feared that if the Germans got atomic bombs before the Allies did, the war might be over in a few weeks. The sense of urgency which Pegram and Urey brought back with them was of great importance.

By the time of submission of the National Academy's third report and the return of Urey and Pegram from England, the general plan of the reorganisation was beginning to emerge. The Academy's report was more conservative than the British report, as Bush pointed out in his letter of Nov. 27, 1941, to President Roosevelt. It was, however, sufficiently optimistic to give additional support to the plan of enlarging the work. The proposed reorganisation was announced at a meeting of the Uranium Section just before the Pearl Harbor attack (Dec. 7, 1941).

So the possibility of obtaining atomic bombs for use in the war was great enough to justify an "all out" effort, and the NDRC Uranium Section (known as the S-l Section) would be  administered directly through an OSRD S-l Section, and later through an OSRD S-l Executive Committee. The membership of the reorganised S-l Section was as follows: J. B. Conant, representative of V. Bush; L. J. Briggs, chairman; G. B. Pegram, vice-chairman; A. H. Compton, program chief; E. O. Lawrence, program chief; H. C. Urey, program chief; Eger Vaughan Murphree, chairman of the separately organised Planning Board; H. T. Wensel, technical aid; and specialists S. K. Allison, James Wakefield Beams, G. Breit, E. U. Condon, and H. D. Smyth.

A Planning Board was also set up for the technical and engineering aspects of the work, for procurement of materials and for construction of pilot plants and full-size production plants. This Planning Board consisted of E. V. Murphree (chairman), Warren K. Lewis, L. W. Chubb, George Oliver Curme, Jr., and Percival C. Keith (head of the Kellex Company).

One aspect of the Manhattan Project that has been generally overlooked by historians was industry's contribution. Virtually overnight, the Manhattan Project created a nationwide ‘factory’ that rivalled General Motors in size and scale despite serious wartime shortages of manpower and materials. The battle on the home front was waged without restraint. Crawford Hallock Greenewalt of the Du Pont Company in Wilmington, Delaware, and Percival C. Keith of the M. W. Kellogg Company in Jersey City, New Jersey (which opened a subsidiary in Manhattan, NY to focus on research for the Manhattan Project), led these efforts.

Some reports go as far as to state that what made the U.S. different from those other countries that had atomic weapons programs, was not its scientific superiority, but it industrial capacity. And it was the U.S. Army that harnessed and directed that effort.  

Contracts for the development of diffusion and centrifuge separation processes were to be recommended by the Planning Board, which would also be responsible for the heavy-water production program. 

The scientific aspects of the work were separated from the procurement and engineering phases. The Program Chiefs H. C. Urey, E. O. Lawrence, and A. H. Compton were to have charge of the scientific aspects. Initially it was proposed that Urey should have charge of the separation of isotopes by the diffusion and the centrifuge methods and of the research work on the production of heavy water. Lawrence was to have charge of the initial production of small samples of fissionable elements, of quantity production by electromagnetic-separation methods, and of certain experimental work relating to the properties of the plutonium nucleus. Compton was to have charge of fundamental physical studies of the chain reaction and the measurement of nuclear properties with especial reference to the explosive chain reaction. He was authorised to explore also the possibility that plutonium might be produced in useful amounts by the controlled chain-reaction method. 

Gaseous diffusion was based on the well-known principle that molecules of a lighter isotope would pass through a porous barrier more readily than molecules of a heavier one, this approach proposed to produce by myriad repetitions a gas increasingly rich in uranium-235 as the heavier uranium-238 was separated out in a system of cascades. Theoretically, this process could achieve high concentrations of uranium-235 but, like the electromagnetic method, would be extremely costly. British researchers led the way on gaseous diffusion, with John Ray Dunning and his colleagues at Columbia University joining the effort in late 1940.


The centrifuge was for many scientists the best hope for isotope separation. The idea was to use a high-speed centrifuge, a device based on the same principle as the cream separator. Centrifugal force in a cylinder spinning rapidly on its vertical axis would separate a gaseous mixture of two isotopes since the lighter isotope would be less affected by the action and could be drawn off at the centre and top of the cylinder. A cascade system composed of hundreds, perhaps thousands, of centrifuges could produce a rich mixture. This method, being pursued primarily by Jesse Wakefield Beams at the University of Virginia, received much of the early isotope separation funding. 

The electromagnetic method, pioneered by Alfred O. Nier of the University of Minnesota, used a mass spectrometer, or spectrograph, to send a stream of charged particles through a magnetic field. Atoms of the lighter isotope would be deflected more by the magnetic field than those of the heavier isotope, resulting in two streams that could then be collected in different receivers. The electromagnetic method as it existed in 1940, however, would have taken far too long to separate quantities sufficient to be useful in the war. In fact, twenty-seven thousand years would have been required for a single spectrometer to separate one gram of uranium-235. Below is the Alpha 1 “racetrack” with the calutrons located around the ring. 

1. Source (cathode block) without in-leakage system, 2. Working substance metal Pd, 3. Arc discharge current (0.5-1.5 A) 4. Arc discharge voltage (150-350 V), 5. Power of graphite crucible heater (2500-4000 W) and above it the ion-optical system to shape beam, 6. Pressure in the (gas-discharge) separating chamber (1-2x10−3 Pa), 7. Pd load in crucible (15-20 g), 8. Mean operating time of the source (25-30 hours), 9. Ionic current on the receiver (15-25 mA), 10. Six box receiver, 11. To vacuum outlet. The magnetic field was 2600 Oersted. 

The effect of the reorganisation was to put the direction of the projects in the hands of a small group consisting of Bush, Conant, Briggs, Compton, Urey, Lawrence, and Murphree. Theoretically, Compton, Lawrence, Urey, and Murphree were responsible only for their respective divisions of the program.

The cost of the initial work was estimated at $4-5 million, and it was stated the Army would take over when full-scale construction was started, presumably when pilot plants were ready.  

In a meeting of the reorganized S-l Section Conant called for an all-out effort given the military value of atomic bombs and that all attention must be concentrated in the direction of  bomb development. 

Following the Pearl Harbor attack on 7 Dec. 1941, the United States entered WW II (Germany and Italy declared war on the U.S. three days later).

In January 1942, President Franklin D. Roosevelt gave secret, tentative approval for the development of an atomic bomb. The Army Corps of Engineers was assigned the task and set up the Manhattan Engineer District to manage the project. A bomb research and design laboratory was to be built at Los Alamos, New Mexico. Due to uncertainties regarding production effectiveness, two possible fuels for the reactors were to be produced with uranium enrichment facilities at Oak Ridge, Tennessee, and plutonium production facilities at Hanford, Washington.

Just before the attack on Pearl Harbor Compton’s committee had concluded that a critical mass of between two and 100 kilograms of uranium-235 would produce a powerful fission bomb and that for $50-100 million isotope separation in sufficient quantities could be accomplished. Bush forwarded the findings to Roosevelt under a cover letter on Nov. 27, 1941. Roosevelt did not respond until January 19, 1942 when he did, it was as commander in chief of a nation at war. The President’s hand-written note read, “V.B.  OK   returned   I think you had best keep this in your own safe   FDR”. 

In 1942 it had been estimated that military and industrial targets in Germany could be devastated with 500,000 tons of TNT bombs, which would be the equivalent to 1 to 10 tons of uranium-235. It was said that Bush had instructed everyone not to discuss the expense of producing uranium-235 to avoid arousing government fears of excessive costs.

As already mentioned, with the written agreement of Roosevelt Bush immediately put Eger V. Murphree, a chemical engineer with the Standard Oil Company, in charge of engineering studies and supervising pilot plant construction and any laboratory-scale investigations. He confirmed Urey, Lawrence, and Compton as program chiefs. Urey headed up work including diffusion and centrifuge methods and heavy-water studies. Lawrence took electromagnetic and plutonium responsibilities, and Compton ran chain reaction and weapon theory programs.

Following this Compton decided to concentrate the work for which he was responsible at the University of Chicago. The Columbia group under Fermi, and its accumulated material and equipment, and the Princeton group, which had been studying resonance absorption, were moved to Chicago in the course of spring 1942. 

Under Lawrence the investigation of large-scale electromagnetic separation was accelerated at the University of California at Berkeley and a related separation project was started at Princeton. 

Research and development on the diffusion process and on the production of heavy water continued at Columbia under Urey, under the general supervision of Murphree. The centrifuge work continued at the University of Virginia under Beams while the Columbia centrifuge work was transferred to the laboratories of the Standard Oil Development Co. at Bayway, New Jersey.

With the entry into War of the U.S. the situation changed from having too little money and no deadlines, to a situation with a clear goal, plenty of money, and too little time. 

On March 9, 1942, Bush sent a report to the President reflecting general optimism but placing proper emphasis on the tentative nature of conclusions. His report contemplated completion of the project in 1944. In addition, the report contained the suggestion that the  Army be brought in during the summer of 1942 for construction of full-scale plants.

The entire heavy-water program was under review in March and April 1942. The reviews followed a visit to the United States in February and March 1942 by Francis Simon, Hans Heinrich von Halban, and Wallace Alan Akers from England. In a memorandum of April 1, 1942 addressed to Bush, Conant reviewed the situation and reported on conferences with Compton and Briggs. His report pointed out that extremely large quantities of heavy water would be required for a plutonium production plant employing heavy water instead of graphite as a moderator. For this reason, he reported adversely on the suggestion that Halban be invited to bring to this country the 165 litres of heavy water which he then had in England.

In a  memorandum written to Bush on May 14, 1942, Conant estimated that there were five separation or production methods which were about equally likely to succeed: the centrifuge, diffusion, and electromagnetic methods of separating uranium-235, and the uranium-graphite pile and the uranium-heavy-water pile methods for producing plutonium. All were considered about ready for pilot plant construction and perhaps even for preliminary design of production plants. If the methods were to be pushed to the production stage, a commitment of $500 million would be entailed. Although it was too early to estimate the relative merits of the different methods accurately, it was presumed that some methods would prove to be more rapid and efficient than others. It was feared, however, that elimination of any one method might result in a serious delay. 

It was also thought that the Germans might be some distance ahead of the United States in a similar program.

Research on uranium required uranium ore, and obtaining sufficient supplies was the responsibility of Murphree and his group. Fortunately, enough ore was on hand to meet the projected need of 150 tons through mid-1944. Twelve hundred tons of high-grade ore were stored on Staten Island, and Murphree made arrangements to obtain additional supplies from Canada and the Colorado Plateau, the only American source. Uranium in the form of hexafluoride was also needed as feed material for the centrifuge and the gaseous and thermal diffusion processes. Philip Abelson, who had moved from the Carnegie Institution to the U.S. Naval Research Laboratory, was producing small quantities, and Murphree made arrangements with E. I. du Pont de Nemours and Company (their TNX Division for the atomic energy program) and the Harshaw Chemical Company of Cleveland to produce hexafluoride on a scale sufficient to keep the vital isotope separation research going.

Uranium metal was also required. Uranium ore is usually refined as uranium oxide, often called black oxide, or uranium salt. In early 1942 only small quantities of uranium metal were available. In fact uranium ore had been stockpiled from a mine in northwest Canada, but the mine had to be re-opened when a large new order for uranium ore was made in 1941. The National Bureau of Standards developed a new process for removing all impurities by a single extraction method, and the Mallinckrodt Chemical Works developed a large-scale production of uranium oxide. This was then converted into uranium tetrafluoride, or green salt, which is the feed material for most uranium metal-making processes. 

Graphite was available in the U.S., but what was needed was a very pure graphite, in particular with no boron. The National Bureau of Standards traced the boron in commercial graphite to the coke used for its production. Coke was substituted with petroleum, which solved the problem. 

Heavy water was another problem. Arrangements had been made post June 1943, but there was an immediate need, and the world’s entire stock of heavy water (approx. 400 pounds) was in the hands of the British. In fact Joliot-Curie had sent his collaborators, Hans von Halban and Lew Kowarski, with 165 litres of heavy water, first to Britain, then to Canada.

There was a major requirement for several thousand tons of copper for the magnet coils used by Lawrence. But copper was a critical war material, so silver was used. Initially when they asked for 15,000 tons of silver from the Under Secretary of the U.S. Treasury, the answer was “we usually talk of silver in ounces”. The first transfer was for 175 million fine troy ounces, i.e. 6,000 tons, but the final figure turned out to 14,700 tons.  

Conant emphasised a question that has been crucial throughout the development of the uranium project. The question was whether atomic bombs would be decisive weapons or merely supplementary weapons. If they were decisive, there was virtually no limit to the amount of effort and money that should be put into the work. But no one knew how effective the atomic bombs would be.

Lawrence proved to be very successful in producing enriched samples of uranium-235 electromagnetically with his converted cyclotron. Bush told the President that Lawrence’s work might lead to a short cut to the bomb, especially in light of new calculations indicating that the critical mass required might well be smaller than previously predicted. Bush also emphasised that the efficiency of the weapon would probably be greater than earlier estimated and expressed more confidence that it could be detonated successfully. 

The President responded: “I think the whole thing should be pushed not only in regard to development, but also with due regard to time. This is very much of the essence”. 

In May 1942, Conant suggested to Bush that instead of encouraging members of the section individually to discuss their own phases of the work with Conant and Briggs, the OSRD S-l Section should meet for general discussions of the entire program. Bush responded by terminating the OSRD S-l Section and replacing it with the OSRD S-l Executive Committee, consisting of the following: J. B. Conant, chairman, L. J. Briggs, A. H. Compton, E. O. Lawrence, E. V. Murphree, H. C. Urey. H. T. Wensel and I. Stewart were selected to sit with the Committee as technical aide and secretary respectively.

On June 13, 1942, Bush and Conant sent to Vice-President Henry A. Wallace, Secretary of War Henry L. Stimson, and Chief of Staff General George Catlett Marshall a report recommending detailed plans for the expansion and continuation of the atomic-bomb program. 

On June 17, 1942, the report was sent by Bush to the President, who also approved its conclusions. 

The report contained four principal parts:

Firstly, it was clear that an amount of uranium-235 or plutonium comprising a number of kilograms would be explosive, that such an explosion would be equivalent to several thousand tons of TNT, and that such an explosion could be caused to occur at the desired instant. 

Secondly, it was clear that there were four methods of preparing the fissionable material and that all of these methods appeared feasible; but it was not possible to state definitely that any given one of these is superior to the others.

Thirdly, it was clear that production plants of considerable size could be designed and built. Fourthly, it seemed likely that, granted adequate funds and priorities, full-scale plant operation could be started soon enough to be of military significance.

It was also noted that if four separate methods all appeared to a highly competent scientific group to be capable of successful application, it appeared certain that the desired end result could be attained by the enemy, provided he had sufficient time. 

Therefore it was unsafe at that time, in view of the pioneering nature of the entire effort, to concentrate on only one means of obtaining the result.

In the meantime, however, isotope separation studies at Columbia quickly confronted serious engineering difficulties. Not only were the specifications for the centrifuge demanding, but, depending upon rotor size, it was estimated that it would require tens of thousands of centrifuges to produce enough uranium-235 to be of value. 

Gaseous diffusion immediately ran into trouble as well. Fabrication of an effective barrier to separate the uranium isotopes seemed so difficult as to relegate gaseous diffusion to a lower priority (the barrier had to be a corrosion-resistant membrane containing millions of submicroscopic holes per square inch). Both separation methods demanded the design and construction of new technologies and required that parts, many of them never before produced, be fitted to tolerances not previously imposed on American industry.

In Chicago, Compton decided to combine all pile research by stages. Initially he funded Fermi’s pile at Columbia and the theoretical work of Eugene Wigner at Princeton and J. Robert Oppenheimer at Berkeley. He appointed Szilárd head of materials acquisition and arranged for Seaborg to move his plutonium work from Berkeley to Chicago in April 1942. Compton secured space wherever he could find it, including a racket court under the West grandstand at Stagg Field, where Samuel K. Allison began building a graphite and uranium pile.

Although it was recognised that heavy water would provide a moderator superior to graphite, the only available supply was a small amount that the British had smuggled out of France. In a decision typical of the new climate of urgency, Compton decided to forge ahead with graphite, a decision made easier by Fermi’s increasingly satisfactory results at Columbia and Allison’s even better results in Chicago. In light of recent calculations that cast doubt on the MAUD report’s negative assessment of plutonium production, Compton hoped that Allison’s pile would provide plutonium that could be used as material for a weapon.

Enrico Fermi (1901-1954) won the Nobel Prize in Physics in 1938 for his work on induced radioactivity, and he was the father of world’s first nuclear reactor, the Chicago Pile-1 (CP-1). 

By May 1942 Bush decided that production planning could wait no longer, and he instructed Conant to meet with the S-1 section leaders and make recommendations on all approaches to the bomb, regardless of cost. 

Analysing the status of the four methods of isotope separation then under consideration,  i.e. gaseous diffusion, centrifuge, electromagnetic, and pile, the committee decided on May 23, 1942 to recommend that all be pushed as fast as possible. This decision reflected the inability of the committee to distinguish a clear front-runner and its consequent unwillingness to abandon any method. With funds readily available and the outcome of the war conceivably hanging in the balance, the S-1 leadership recommended that all four methods proceed to the pilot plant stage and to full production planning.

With five methods of producing fissionable material, three isotope separation processes (electromagnetic, gaseous diffusion and centrifuge) for uranium-235, and two piles (uranium-graphite and uranium-heavy water) for plutonium, Conant called it a “five horse race”. By Feb. 1942, 10 contracts had been made with 12 institutions for a total of more than $1 million, and that figure was doubled in March 1942. In fact they had already decided to allocate between $10 and $17 million for all five ‘horses' through to early 1943. 

The Manhattan District

On June 18, 1942, Colonel James Creel Marshall, Corps of Engineers, was instructed by the Chief of Engineers to form a new district in the Corps of Engineers to carry out “special work” (atomic bombs) assigned to it. This district was designated the Manhattan District and was officially established on Aug. 13, 1942. The work with which it was concerned was labeled, for security reasons, the "DSM Project" (Development of Substitute Materials).

Marshall was told to form a new engineering district “for construction of a new manufacturing plant” as part of a project already in progress to develop atomic energy for military purposes. In fact both the U.S. Army and Navy had been briefed by Sachs on Oct. 12, 1939, but ironically, the technical service that eventually had the most to do with the development of the atomic bomb, the Corps of Engineers, was not consulted. In the meeting in 1939 the Army was not enthusiastic, and was even negative about the military potential of such a development. Colonel Marshall was reassigned (Aug. 1943) as commanding officer of the Engineer Training Centre, and was promoted to Brigadier General (he later served in Australia, New Guinea and the Philippines). Another report was more blunt and simply said that following frustrating project delays, General George C. Marshall and Lieutenant General Brehon Burke Somervell, replaced Colonel Marshall with a more dynamic officer, Colonel Leslie Groves, on Sept. 17, 1942.

Bush transferred the responsibility for process development, materials procurement, engineering design, and site selection to the Corps of Engineers, and earmarked approximately sixty percent of the proposed 1943 budget, or $54 million.

An Army officer would be in overaIl command of the entire project. This new arrangement changed the U.S. atomic bomb effort from one dominated by research scientists to one run by the U.S. Army (despite the fact that they knew nothing about nuclear physics).

At the time Marshall was directly subordinate to the Engineers chief, Brig. Gen. Thomas M. Robins, and worked with his deputy Col. Leslie R. Groves. It was Marshall who recruited the core team and set up the liaison office in Washington, D.C. It was Marshall who suggesting using “Development of Substitute Materials” (DSM) as a name, but finally, with Groves, they decided on calling it “Manhattan” (normally Engineer districts took their names for the city were they were located). Manhattan, in New York, was where Marshall had established his temporary headquarters.

It has been written that at this time S-1 and the Manhattan District were free to act on any mutually agreed decision. Site selection became important in their meeting of June 25, 1942. They wanted to pick one site for all industrial-scale construction, at least 200 miles from national borders, with 150,000 kilowatts of electricity available, and hundreds of thousands of gallons of water per minute also available. In addition it should allow construction in winter, have access to a ready supply of labour, good transport access, and have a terrain cut up by ridges to limit the effects of any accidental explosion. A site near Knoxville, Tennessee had been already suggested, the so-called Tennessee Valley (115,000 acres for $3.5 million). It about this time that they also selected a site in the Argonne Forest. And about the same time it was decided that M. W. Kellogg Company would also work on building the diffusion plant. They in fact created a wholly owned subsidiary, the Kellex Corporation, and a subsidiary of Union Carbide was contracted to operate the diffusion plant. 

Objectives, set in June 1943, were that by Jan. 1944 they would have a full-scale centrifuge plant producing about 1 kilogram of enriched uranium-235 per day. The diffusion plant should be a 1-kilogram-per-day production facility, and the electromagnetic process should be producing 100-grams-per-day. They also wanted a plutonium plant producing 100-grams-per-day, with a heavy water plant producing 0.5 tons per month. 

There were major problems to overcome, despite feasibility being demonstrated. For example, in the diffusion process the barrier needed to be porous to uranium hexafluoride through thousands of stages, and resist the exceptional corrosiveness of the gas.

It was just after the meeting of June, 27, 1942 that it was decided that Stone and Webster would be the managing contractor for the atomic energy project. They would oversee and subcontract all the R&D, procurement, engineering, and construction. 

Another problem was deciding the priority of the Manhattan District. Major military programs received a rating AA-1 to AA-4, with highest being A-1-a (AAA existed for emergencies). The Manhattan District obtained a rating of AA-3 (AA-1 and AA-2 were usually limited for airplanes, ships, guns, etc.), with an optional AAA to “pry loose certain critical items”. This did cause problems, since steel became virtually impossible to obtain without a AA-2 priority. Without a higher priority the atomic bomb program became “an unimportant miscellaneous type”. The District did finally get a blank check to assign the AAA priority as needed, and was also allocated a new priority AA-2X for urgent foreign and domestic industrial programs. In 1943 the District was using more AAA priorities than the total for all Army and non-Army programs combined. The project finally was allocated a full AA-1 priority on July 1, 1944. 

On Sept. 17, 1942, the Secretary of War placed Brigadier General L. R. Groves of the Corps of Engineers in complete charge of all Army activities relating to the DSM Project. 

At the time of his nomination Groves was a Colonel, he was later promoted to Brigadier General (in fact his nomination was held back until he obtained his promotion). Groves was an engineer who had been involved in building the Pentagon. Groves had already been advising the District Engineer Marshall concerning power resources and site selection for the Manhattan District facilities. 

Groves acted immediately, buying a site in Tennessee, and moving the Manhattan Engineering District to Washington. He also imposed on the scientists a need to select, by late 1942, the best isotope separation technologies for full-scale commitment. 

Berkeley, with Lawrence and the electromagnetic technique, remained the frontrunner. Lawrence refined his 184-inch magnet and huge cyclotron to produce calutrons. The 184-inch cyclotron magnet (see below) was nearly five times wider that the 37-inch magnet used in Lawrence’s previous experiments, and had been totally funded by the Rockefeller Foundation. The recommendation was now to build a pilot plant and full-scale plant on the Tennessee site. 

The centrifuge was the big loser. Westinghouse had found it just too difficult to stop severe vibrations during trial runs.

Gaseous diffusion remained alive, but the problem was still the barrier. 

The pile at Stagg Field looked promising, but there was a constant shortage of graphite and uranium. On the other hand Seaborg had developed a way to separate plutonium from irradiated uranium using plutonium’s different oxidation states. In Aug. 1942 he had produced a microscopic sample of pure plutonium. It was even decided to build a production facility at Site X at Clinch River in Tennessee. Groves wanted Du Pont to take on this challenge, but they felt that the mass production of plutonium could only be achieved by 1945. 

Glenn Theodore Seaborg (1912-1999) won the Nobel Prize for Chemistry in 1951 for his discovery of ten transuranium elements (including the actinides).   

Site X was the original name given to the 59,000 acres of land along the Clinch River, it was only after the WW II that the name was changed to Oak Ridge.  

On Aug. 4, 1944, an incident occurred with the plutonium chemist Don Mastick at Los Alamos. Liquid plutonium in a vial had undergone an unanticipated transformation overnight. Some of the liquid became a gas and was ingested by Mastick. At the time, the medical doctor at Los Alamos was Louis Hempelmann. He was only 29 years old, and like Manhattan’s Dr. Alderman, he had little understanding of the effect of radiation on the human body. After hearing Mastick’s account, Hempelmann made a frantic call to Stafford Leak Warren, the medical director of the Manhattan Project (and inventor of the mammogram). Warren was an Army colonel who worked at the project’s administrative headquarters in Oak Ridge, TN, and who was responsible for conducting research on the health effects of radiation exposure (and reported directly to General Groves). Warren told Hempelmann to use a mouthwash, expectorant and stomach pump to remove the plutonium. So Mastick was handed a four-litre beaker of “murky liquid”, and then, as the resident plutonium chemist, Mastick was later responsible for retrieving the plutonium from his own fluids. 

The reality is that the ingestion of plutonium does not cause adverse health effects. Manhattan Project chemist Lawrence Bartell had a similar experience while performing an experiment with plutonium at the University of Chicago. “One of the things that we had to do was to pipette solutions to get the right amount to mix and to precipitate”, recalls Bartell, and “one day the pipette backed up and I found myself with plutonium in my mouth”. Bartell, who was embarrassed by his mistake and was too ashamed to go to the health physics division, rinsed out his mouth as best he could and went about his day. According to Bartell, plutonium “goes through the body fairly fast if taken orally”. If it gets into your lungs, however, “you’re in very bad trouble”.

Plutonium is toxic, both chemically and because of its ionising radiation, but there are many substances that are more toxic (kg for kg), such as arsenic, cyanide, etc. If you were to eat (ingest) plutonium it would pass through the gastro-intestinal tract and be expelled without causing much harm. The main danger comes from inhalation, particularly with particles sizes less than 0.01 mm. Most particles would be be exhaled or expelled by the mucous flow from the bronchial system into the gastro-intestinal tract. But some particles would be trapped and find their way through the lungs into the blood or lymph system, and to the bones or liver. These deposited plutonium particles are alpha emitters and could cause cancer.   

In the late 1930s, Du Pont had introducing nylon, the first synthetic polymer. Behind this success was a new technique for continuous operations with the ingredients at one end and the product at the other. The process was revolutionary, a vast improvement over earlier step-by-step approaches. Groves was convinced that Du Pont could apply the same ingenuity to the plutonium production process at Hanford and tried to persuade Du Pont to join the Project at the end of Oct. 1942. Despite Groves' urging, it took a call from President Roosevelt to convince Du Pont to accept the task.  

In fall 1942 Robert Oppenheimer, leading a group of theoretical physicists, suggested that the critical mass needed would be about twice that initially expected. However they also reported that fusion explosions using deuterium (heavy hydrogen) might be possible. The possibility of thermonuclear (fusion) bombs generated some optimism since deuterium supplies, while not abundant, were certainly larger and more easily supplemented than were those of uranium and plutonium. Basic research on other light elements was authorised. 

In December 1942, Roosevelt gave final approval to construct a nuclear bomb. 

So the centrifuge project was cancelled, and gaseous diffusion (lower priority), the pile, and electromagnetic separation were to proceed to full-scale development, eliminating the pilot project stage. In Nov. 1942 the Lewis Committee elevated gaseous diffusion to first priority. 

World’s first self-sustaining chain reaction

At 3:20 p.m. on Dec. 2, 1942, Fermi’s massive lattice pile of 400 tons of graphite, six tons of uranium metal, and fifty tons of uranium oxide achieved the first self-sustaining chain reaction, operating initially at a power level of one-half watt (increased to 200 watts ten days later).  

This is the only original photograph of the reactor being built.

As Compton reported to Conant, “the Italian navigator has just landed in the new world”. To Conant’s question, “Were the natives friendly?” Compton answered, “Everyone landed safe and happy”. Significant as this moment was in the history of physics, it had already been decided to move to the pilot and Groves had already instructed Du Pont to move into design and construction on Dec. 1, 1942.

For the test, most people were on the balcony of the court. Norman Hilberry, equipped with an axe, was prepared to sever a rope tied to balcony rail, which would drop into place an emergency safety rod suspended over pile (Hilberry went on to direct the Argonne National Laboratory). George Leon Weil handled the final control rod. On the platform above the pile, three men stood ready to flood it with cadmium salt solution, which would absorb sufficient neutrons to halt a runaway reaction (they were called “the suicide squad”). Also in the control room they had command of the safety rods. As the last control rod was removed, foot-by-foot, tests were performed. Halfway through Fermi said he was hungry, and they broke for a lunch lasting a little more than two and one-half hours. They continued removing the control rod shortly after 14:00, and declared that the reaction was self-sustaining at 15.25. Wine from a now famous flask of Chianti was poured into a paper cup and passed around. No toast, no remarks, nothing.  

On Dec. 28, 1942, Roosevelt approved an investment of $500 million to build full-scale gaseous diffusion and plutonium plants and the compromise electromagnetic plant (smaller than the others), as well as heavy water production facilities.

The procurement and engineering functions of the Planning Board were taken over by the Manhattan District in the summer of 1942 and that by the spring of 1943 the Manhattan District took over the research and development contracts from the OSRD. Such a transfer was effected as of May 1, 1943, and marked the end of the formal connection of OSRD with the uranium project.

In many way the Manhattan Engineering District had become a big construction project. It purchased and prepared sites, let contracts, hired personnel and subcontractors, built and maintained housing and service facilities, placed orders for materials, developed administrative and accounting procedures, and established communications networks. 

By the end of the war Groves and his staff had spent approximately $2.2 billion on production facilities and towns built in the states of Tennessee, Washington, and New Mexico, as well as on research in university laboratories from Columbia to Berkeley.

What made the Manhattan Project unlike other companies performing similar functions was that, because of the necessity of moving quickly, it invested hundreds of millions of dollars in unproven and hitherto unknown processes and did so entirely in secret. Inherent to this approach was what is now often called trial-and-learning experimentation, a way to mix problem solving and innovation. Also called “experimentation in the unknown”, the focus was (and still is today for many companies) on inventing something while dealing with incomplete knowledge or even profound ignorance. This type of experimentation is often used when there are several different problems to solve, when there are many alternatives to test, and when the evaluation criteria are not pre-given. When the Manhattan Project started the fundamental principles were clear, but theories were full of unverified assumptions. In some cases no theories were available, so experimentation (trial and error) was the only solution. Groves called it “proceeding in the dark”. For example, the work on electromagnetic separation was all about magnetic shims, sources, and collectors that gave the best results, although no one could explain why one combination was better than another. Perhaps the most obvious example was the development of the implosion technique. The starting point, in July 1943, were incredibly crude experiments using TNT surrounding hollow steel cylinders. It was John Von Neumann, in Sept. 1943, that suggested arranging shaped charges in a spherical configuration around the active material, and aim for a faster kind of implosion based upon increasing the amount of high explosive. The final assembly consisted of a 5-foot diameter sphere composed of ‘lens’ for a total of nearly 3 tons of high explosive. The theory work only started in Jan. 1944, with the analysis of E. Teller on the (shock wave) hydrodynamics of implosions. Calculations were impossible by hand, and were at the limit of the computing power available at that time. This was one of the reasons why Los Alamos became a pioneer in using computers (they received a big IBM machine in April 1944). In 1944 there were analytic solutions for shock waves in perfect gases, but nothing for nuclear materials in a temperature-pressure-energy region of interest in an atomic bomb. Despite that, Teller was able to show that an implosion was possible, and he was able to define the kinds of experiments needed. The impact of these initial calculations and experiments was that two new divisions, “Gadget” G Division and Explosives X Division, were created. Throughout the first half of 1944 the focus was on how to measure (extremely fast) implosion parameters such as symmetry, time of collapse, and degree of compression. They ended up identifying seven different approaches, ranging from X-ray photography, through the RaLa method, to a so-called betatron method. For example, the X-ray photography revealed “jet” formations during implosion, and at the same time pushed the development of very precise timing circuits. In this particular example, the RaLa method, invented by Robert Serber in Nov. 1943, involved placing a gamma-ray source in the centre of the spherical assembly. The gamma-rays would travel out radially, and where affected by the density changes in the collapsing sphere of metal. The data provided time of collapse, and degree of symmetry and compression. Nice idea, but they then had to find a good gamma-source (so-called RadioLantanum-140) and design new ionisation chambers (that would be destroyed in the explosion), new recording systems, and build a test site with a bomb-proof shelter. Just getting RadioLantanum-140 required Oak Ridge to build a special extraction laboratory and a second plant for dissolving irradiated uranium slugs and recovering barium from the solution (and then secretly ship it 1,200 miles to Los Alamos). RadioLantanum-140 was the most powerful radiation source people had worked with at that time. Experiments started Sept. 22, 1944, and only on Dec. 14, 1944 did it really work properly. Electronic detonators appeared in Feb. 1945, along with the first observation of sizeable compression. And by June 1945, they had a viable design for the explosive 'lenses'. Some experts have summarised this entire process as “blowing things up and looking to see what happened”. There was no established art for implosion devices. Experiments were the only option with no “a priori” knowledge. And each experiment lead to new questions and new problems, which lead to new multi-disciplinary groups being formed. And each experiment required a new generation of measurement instruments (the experiments were not possible with existing technology). A key (hidden) concept seen in this example, was to tell the difference between failures and mistakes. Mistakes were wrong experiments that did not help in the understanding of a problem, failures were to be learned from and pointed to improvements needed. The reality was that the implosion example had both, so it was important to understand the difference and learn (in different ways) from both. In some cases there was no way to tell the difference between a mistake and a failure, so the key was to “amass a body of understanding” through a series of overlapping experiments, none of which would individually have been able to provide the solution. 

One nice 'general' rule that has been identified is that if you are really looking at a problem that requires radical innovation, then the available instruments of measurement and test are almost always inadequate, or even inexistent. 

For the Manhattan Project the need for haste clarified priorities and shaped decision making. Unfinished research on three separate, unproven processes had to be used to freeze design plans for production facilities, even though it was recognised that later findings inevitably would dictate changes. The pilot plant stage was eliminated entirely, violating all manufacturing practices and leading to intermittent shutdowns and endless troubleshooting during trial runs in the production facilities.

In July 1943 Conant and Richard G. Tolman were formally asked by General Groves to serve as his scientific advisers. They had already been doing so informally and continued to do so. Coordination of the various scientific and technical programs was accomplished by meetings between General Groves and the leaders of the various projects, in particular, Compton, Lawrence, Oppenheimer, and Urey.

Oak Ridge

Work was on-going on the East Tennessee site where the first production facilities were to be built. Late 1942 saw the acquisition of the roughly ninety-square-mile parcel (59,000 acres) in the ridges just west of Knoxville, and extensive site preparation to provide the transportation, communications, and utility needs of the town and production plants that would occupy the previously underdeveloped area. Original plans called for the Clinton Engineer Works, as the military reservation was named, to house approximately 13,000 people in prefabricated housing, trailers, and wood dormitories. 

Aerial View of the town of Oak Ridge looking east. In September 1942, General Leslie Groves, U.S. Army Corps of Engineers, designated a 59,000-acre parcel of land between Black Oak Ridge to the north and the Clinch River to the south as the first federal reserve for producing nuclear materials for the atomic bomb. Framed by the foothills of the Appalachian Mountains, the site originally consisted of 3,000 residents on family farms and in small communities. 

The Manhattan Engineer District headquarters were moved from Washington to Tennessee in the summer of 1943 (Groves kept the Manhattan Project’s office in Washington). 

It was estimated that the town of Oak Ridge would be home to 40-45,000 people, but by May 1945, it was home to 82,000 people. (The name Oak Ridge did not come into usage until after WW II, but will be used here to avoid confusion). At the end of the war, Oak Ridge was the fifth largest town in Tennessee, and the Clinton Engineer Works was consuming one-seventh of all the power being produced in the U.S. The three production facility sites were located in valleys away from the town. This provided security and containment in case of explosions. The Y-12 area, home of the electromagnetic plant, was closest to Oak Ridge, being but one ridge away to the South. Farther to the south and west lay both the X-10 area, which contained the experimental plutonium pile and separation facilities, and K-25, site of the gaseous diffusion plant and later the S-50 thermal diffusion plant. Y-12 and X-10 were begun early in 1943, and K-25 slightly later, but all three were well along by the end of the year. Below we can see Y-12 (left) and X-10 (right) in 1944.  

Although the Lewis report had placed gaseous diffusion ahead of the electromagnetic approach, many were still betting in early 1943 that Lawrence and his mass spectrograph would eventually predominate. Lawrence and his laboratory of mechanics at Berkeley continued to experiment with the giant 184-inch magnet, trying to reach a consensus on which shims, sources, and collectors to incorporate into Y-12 design for the Oak Ridge plant. Research on magnet size and placement and beam resolution eventually led to a “racetrack” configuration of two magnets with forty-eight gaps containing two vacuum tanks each per building, with ten buildings being necessary to provide the 2,000 sources and collectors needed to separate 100 grams of uranium-235 daily. It was hoped that improvements in calutron design, or placing multiple sources and collectors in each tank, might increase efficiency and reduce the number of tanks and buildings required, but experimental results were inconclusive even as Stone & Webster of Boston, the Y-12 contractor at Oak Ridge, prepared to break ground.

Tennessee Eastman Corporation (a subsidiary of Eastman Kodak) was the plant operator of Y-12 and the electromagnetic equipment was manufactured by the Westinghouse Electric and Manufacturing Company, the Allis-Chalmers Manufacturing Company, and the Chapman Valve Manufacturing Company. General Electric agreed to provide electrical equipment. 

On Jan. 14, 1943 the requirement was for the first racetrack of 96 tanks to be finished by July 1, 1943, and all 500 tanks finished by end 1943. Each racetrack was 122 feet long, 77 feet wide, and 15 feet high, the actual tank design was still in flux, and there were no plans yet for the chemical extraction facilities. 

On March 17, 1943, it was decided to separate the process into two stages. The purpose of the second stage was to take the enriched uranium-235 derived from several runs of the first stage and use it as the sole feed material for a second stage of racetracks containing tanks approximately haIf the size of those in the first. Groves approved this arrangement and work began on both the Alpha (first-stage) and Beta (second-stage) tracks.

Building the Alpha plant was started on Feb. 18, 1943, and the Beta plants was started before the decision was made to adopt a 2-stage process. Huge amounts of material had to be obtained (38 million broad feet of lumber, for instance), and the magnets needed so much copper for windings that the Army had to borrow almost 15,000 tons of silver bullion from the U.S. Treasury to fabricate into strips and wind on to coils as a substitute for copper. Treasury silver was also used to manufacture the busbars that ran around the top of the racetracks. Below we have the Alpha (left) and smaller Beta (right). 


Improvements and changes were made on the fly. Lawrence found that hot (high positive voltage) electrical sources could replace the single cold (grounded) source, providing more efficient use of power, reducing insulator failure, and making it possible to use multiple rather than single beams. Meanwhile, receiver design evolved quickly enough in spring and summer 1943 to be incorporated into the Alpha plant. For a variety of reasons, including simplicity of maintenance, Tennessee Eastman decided that the Beta plant would consist of a rectangular, rather than oval, arrangement of two tracks of thirty-six tanks each.

About the same time Oppenheimer, in his new isolated laboratory in Los Alamos, warned that one might need three times as much fissionable material as had been originally indicated. So the designed Alpha and Beta racetracks might not do the job. Lawrence added additional beams to the design (4-beam instead of 1-beam), and convinced Groves to double the size of the complex with two new buildings, each with two rectangular racetracks of 96 tanks operating with 4-beam sources. 

Along with the building at Oak Ridge, 5,000 people were also recruited to operate and maintain the new facilities. Vacuum tanks in the first Alpha racetrack leaked and shimmied out of line due to magnetic pressure, welds failed, electrical circuits malfunctioned, and operators made frequent mistakes. Most seriously, the magnet coils shorted out because of rust and sediment in the cooling oil. 

By Feb. 1944 Y-12 had only produced 200 grams of 12% enriched uranium. And K-25, the gaseous diffusion plant, still had problems with the barriers. K-25 would not be able to produce sufficiently enriched material for Y-12, without it being redesigned and retooled. An additional 4 racetracks would also be need, in addition to the 9 already finished or under construction.

If was finally decided to throw everything behind a new 30-beam calutron design. 

From 1943 General Groves continued to carry the major responsibility for correlating the whole effort and keeping it directed toward its military objectives. His duty was to keep the various parts of the project in step, to see that raw materials were available for the various plants, to determine production schedules, to make sure that the development of bomb design kept up with production schedules, to arrange for use of the bombs when the time came, and to maintain an adequate system of security. Brigadier General T. F. Farrell acted as General Groves' deputy in the important later phases of the project. Colonel Kenneth David Nichols was the District Engineer of the Manhattan District with his headquarters at the Clinton Engineer Works. He was concerned with the research and production problems of both uranium-235 and plutonium. Colonel Franklin Thompson Matthias was responsible for both the construction and operational phases at the Hanford Engineer Works. Colonel Warren was the chief of the Medical Section of the Manhattan District.

Here the story continues more or less word-for-word from “The Manhattan Project: Making the Atomic Bomb”, by F.G. Gosling (1999), report DOE/MA-0001-01/99.  

Eleven miles southwest of Oak Ridge on the Clinch River was the site of the K-25 gaseous diffusion plant. Championed by the British and placed first by the Lewis committee, gaseous diffusion seemed to be based on sound theory but had not yet produced samples of enriched uranium-235. At Oak Ridge, on a relatively flat area of about 5,000 acres, site preparation for the K-25 power plant began in June, 1943. Throughout the summer, contractors contended with primitive roads as they shipped in the materials needed to build what became the world’s largest steam electric plant. In Sept. 1943 work began on the cascade building (below), plans for which had changed dramatically since that spring.

K-25 is the original gaseous diffusion plant, and at the time of its construction it was the world’s largest building. K-27, K-29, K-31, and K-33 were additional gaseous diffusion plants added after WW II. The plants ceased to operate in 1985, and up for demolition in 2016.

Now there were to be fifty four-story buildings (2,000,000 square feet) in a U-shape measuring half a mile by 1,000 feet. Innovative foundation techniques were required to avoid setting thousands of concrete piers to support load-bearing walls. 

Since it was eleven miles from the headquarters at Oak Ridge, the K-25 site developed into a satellite town. Housing was supplied, as was a full array of service facilities for the population that reached 15,000. Dubbed Happy Valley by the inhabitants, the town had housing similar to that in Oak Ridge. 

In late summer 1943 it was decided that K-25 would play a lesser role than originally intended. Instead of producing fully enriched uranium-235, the gaseous diffusion plant would now provide around 50% enrichment for use as feed material in Y-12. This would be accomplished by eliminating the more troublesome upper part of the cascade. Even this level of enrichment was not assured since a barrier for the diffusion plant still dld not exist. The decision to downgrade K-25 was part of the larger decision to double Y-12’s capacity and it fit with Groves’s new strategy of utilising a combination of methods to produce enough fissionable material for bombs as soon as possible.

There was no doubt in Groves’s mind that gaseous diffusion still had to be pursued vigorously. Not only had major resources already been expended on the program, but there was also the possibility that it might yet prove successful. Y-12 was in trouble as 1944 began, and the plutonium pile projects were just getting underway. A workable barrier design might put K-25 ahead in the race for the bomb. Unfortunately, no one had been able to fabricate a barrier of sufficient quality. The only alternative remaining was to increase production enough to compensate for the poor performance of the barrier.

The problems mounted up

So there were problems with both Y-12 and K-25. President Roosevelt had instructed that the atomic bomb effort be an Army program and that the Navy be excluded from deliberations. Navy research on atomic power, conducted primarily for submarines, received no direct aid from Groves, who, in fact, was not up-to-date on the state of Navy efforts when he received a letter on the subject from Oppenheimer late in April 1944.

Oppenheimer informed Groves that Philip Abelson’s experiments on thermal diffusion at the Philadelphia Naval Yard deserved a closer look. The liquid thermal diffusion process had been evaluated in 1940 by the Uranium Committee, when Abelson was at the National Bureau of Standards. In 1941 he moved to the Naval Research Laboratory, where there was more support for his work. During summer 1942 Bush and Conant received reports about Abelson’s research but concluded that it would take too long for the thermal diffusion process to make a major contribution to the bomb effort, especially since the electromagnetic and pile projects were making satisfactory progress. So Abelson continued his work independently of the Manhattan Project. Groves immediately saw the value of Oppenheimer’s remarks, and a quick analysis demonstrated that a thermal diffusion plant could be built at Oak Ridge and placed in operation by early 1945. The steam needed in the convection columns was already at hand in the form of the almost completed K-25 power plant. It would be a relatively simple matter to provide steam to the thermal diffusion plant and produce enriched uranium, while providing electricity for the K-25 plant when it was finished. Groves gave the contractor, H. K. Ferguson Company of Cleveland, just ninety days from Sept. 27, 1943 to bring S-50 (below), a 2,142-column plant on line (Abelson’s plant contained 100 columns).

S-50 became the first stage of enrichment, achieving enrichment levels of less than 2%. This material was fed into K-25 which achieved about 23% enrichment, and this was fed to the calutrons in Y-12, which achieved about 84% enrichment.

The production pile for plutonium

The Metallurgical Laboratory (Met Lab) in Chicago was counted on to design a production pile for plutonium. Here again the job was to design equipment for a technology that was not well understood even in the laboratory. The Fermi pile, important as it was historically, provided little technical guidance other than to suggest a lattice arrangement of graphite and uranium. Any pile producing more power than the few watts generated in Fermi’s famous experiment would require elaborate controls, radiation shielding, and a cooling system. These engineering features would all contribute to a reduction in neutron multiplication (neutron multiplication being represented by k); so it was imperative to determine which pile design would be safe and controllable and still have a k high enough to sustain an ongoing reaction.

A group headed by Compton’s chief engineer, Thomas V. Moore, began designing the production pile in June 1942. Moore’s first goals were to find the best methods of extracting plutonium from the irradiated uranium and for cooling the uranium. It quickly became clear that a production pile would differ significantly in design from Fermi’s experimental reactor, possibly by extending uranium rods into and through the graphite next to cooling tubes and building a radiation and containment shield. Although experimental reactors like Fermi’s did not generate enough power to need cooling systems, piles built to produce plutonium would operate at higher power levels and require coolants. The Met Lab group considered the full range of gases and liquids in a search to isolate the substances with the best nuclear characteristics, with hydrogen and helium standing out among the gases and water (even with its marginal nuclear properties and tendency to corrode uranium) as the best liquid.

During the summer, Moore and his group began planning a helium-cooled pilot pile for the Argonne Forest Preserve near Chicago, built by Stone & Webster, and on Sept. 25, 1942, they reported to Compton. The proposal was for a 460-ton cube of graphite to be pierced by 376 vertical columns containing twenty-two cartridges of uranium and graphite. Cooling would be provided by circulating helium from top to bottom through the pile. A wall of graphite surrounding the reactor would provide radiation containment, while a series of spherical segments that gave the design the nickname “Mae West” would make up the outer shell.

By the time Compton received Moore’s report, he had two other pile designs to consider. One was a water-cooled model developed by Eugene Wigner and Gale Young, a former colleague of Compton’s. Wigner and Young proposed a twelve-foot by twenty-five-foot cylinder of graphite with pipes of uranium extending from a water tank above, through the cylinder, and into a second water tank underneath. Coolant would circulate continuously through the system, and corrosion would be minimised by coating interior surfaces or lining the uranium pipes.

A second alternative to “Mae West” was more daring. Szilárd thought that liquid metal would be such an efficient coolant that, in combination with an electromagnetic pump having no moving parts (adapted from a design he and Einstein had created), it would be possible to achieve high power levels in a considerably smaller pile. Szilárd had trouble obtaining supplies for his experiment, primarily because bismuth, the metal he preferred as the coolant, was rare.

In Oct. 1942 Groves was in Chicago ready to force a showdown on pile design. Szilárd was noisily complaining that decisions had to be made so that designs could move to procurement and construction. Compton’s delay reflected uncertainty on the superiority of the helium pile and awareness that engineering studies could not be definitive until the precise value of k had been established. Some scientists at the Met Lab urged that a full production pile be built immediately, while others advocated a multi-step process, perhaps beginning with an externally cooled reactor proposed by Fermi.

On Oct. 5, 1943 Groves insisted that the Met Lab decide on pile design within a week. Even wrong decisions were better than no decisions, Groves claimed, and since time was more valuable than money, more than one approach should be pursued if no single design stood out. Compton decided on compromise. Fermi would study the fundamentals of pile operation on a small experimental unit to be completed and in operation by the end of 1943. Hopefully he could determine the precise value of k and make a significant advance in pile engineering possible. An intermediate pile with external cooling would be built at Argonne and operated until June 1, 1943, when it would be taken down for plutonium extraction. The helium cooled “Mae West”, designed to produce 100 grams of plutonium a day, would be built and operating by March 1944. Studies on liquid-cooled reactors would continue, including Szilárd’s work on liquid metals.

In practical terms, today there are two different kinds of plutonium: reactor-grade and weapons-grade. The first is recovered as a by-product of used fuel from a nuclear reactor, after the fuel has been irradiated ('burned') for about three years. The second is made specially for the military purpose, and is recovered (separated or reprocessed) from uranium fuel that has been irradiated for only 2-3 months in a plutonium production reactor. The two types of plutonium differ in their isotopic composition, but both are proliferation risks.

The most common isotope formed in a typical nuclear reactor is the fissile plutonium-239 isotope, formed by neutron capture from uranium-238 (followed by beta decay), and which yields much the same energy as the fission of uranium-235. Well over half of the plutonium created in the reactor core is 'burned' in situ and is responsible for about one third of the total heat output of a typical “light water” power reactor (LWR). Of the rest, about one sixth through neutron capture becomes plutonium-240 (and plutonium-241). The approximately 1.15% of plutonium in the spent fuel removed from a commercial LWR power reactor (burn-up of 42 GWd/t) consists of about 53% plutonium-239, 25% plutonium-240, 15% plutonium-241, 5% plutonium-242 and 2% of plutonium-238, which is the main source of heat and radioactivity. 

In a reactor producing weapon plutonium, the burn-up might be about 0.4 GWd/t (so about 3 months irradiation and not 3 years), and a typical plutonium isotope vector might be about 80% plutonium-239, 16.9% plutonium-240, 2.7% plutonium-241, 0.3% plutonium-242 and 0.1% of plutonium-238.

Today a “typical” 1000 MWe light water reactor produces about 25 tons of used fuel a year, containing up to 290 kilograms of plutonium. If the plutonium is extracted from used reactor fuel it can be used as a direct substitute for uranium-235 in the usual fuel, the plutonium-239 being the main fissile part, but plutonium-241 also contributing. In order to extract it for recycle, the used fuel is reprocessed and the recovered plutonium oxide is mixed with depleted uranium oxide to produce a so-called mixed-oxide (MOX) fuel, with about 8% Pu-239 (this corresponds with uranium enriched to 5% U-235). This plutonium can also be used fast breeder reactors, and is the preferred fuel for the future IV generation nuclear reactors

As a best estimate, total world generation of reactor-grade plutonium in spent fuel is some 70 tons per year. About 1300 tons have been produced so far, and most of this remains in the used fuel, with some 370 tons having been extracted. About one third of the separated plutonium (130 tons) has been used in MOX fuel over the last 30 years. Over 20 tons of reactor-grade plutonium is separated by reprocessing plants in the OECD each year and this is set to increase. Eventually its usage in MOX is expected to outstrip this level of production so that stockpiles diminish.

At the end of 2013 the UK plutonium stockpile had 123 tons of separated civil plutonium from historic and current operations and foreign swaps. It includes some 83 tons from Magnox fuel, 15 tons from AGR fuel and 15 tons foreign-owned. On completion of reprocessing operations about 2016 the stockpile is expected to be 140 tons. Using all of UK's plutonium in MOX fuel rather than immobilising it as waste is expected to yield a £700-1200 million resource cost saving to UK, along with over 700 billion kWh of electricity (about two years' UK supply). 

At the end of 2010 France held 80 tons of separated civil plutonium, 60 tons of it at La Hague. Some 10.5 tons of plutonium and 1000 tons of reprocessed uranium  are recovered each year from the 1050 tonnes treated each year. The plutonium is immediately shipped to the 195 tons/yr Melox plant near Marcoule for prompt fabrication into about 100 tonnes of mixed-oxide (MOX) fuel.

Russia holds at least 32 tons from reprocessing power reactor fuel (and 34 tons of weapons-grade plutonium from military stockpiles to be used in MOX fuel for BN-600 and BN-800 fast neutron reactors at Beloyarsk, supported by a $400 million payment from the USA.) 

The U.S. has no reactor-grade plutonium separated, but at least 34 tons of weapons-grade material is destined for MOX. China has no reactor-grade plutonium separated. India’s plutonium stocks are unknown. Worldwide stocks of civil plutonium are estimated as around 260 tons.

Continued disarmament is likely to give rise to some 150-200 tons of weapons-grade plutonium, over half of it in Russia. Most of this is likely to be used in MOX for existing or fast neutron reactors. In June 2000, the U.S. and Russia agreed to dispose of 34 tons each of weapons-grade plutonium by 2014, and since then the U.S. government has released further surplus weapons plutonium (the 2000 agreement was reaffirmed in 2010). Construction on the Mixed Oxide Fuel Fabrication Facility at the Savannah River Site near Aiken, South Carolina commenced in Aug. 2007 (it has a operational license since 2014). The plant is designed to convert 3.5 tons/yr of weapons-grade plutonium into mixed oxide (MOX) fuel. Initial trials of MOX fuel made with weapons plutonium have been successful. Russia plans to continue to use all its military plutonium in fast-neutron reactors.

Transuranium chemistry

While the Met Lab labored to make headway on pile design, Seaborg and his coworkers tried to gain enough information about transuranium chemistry to insure that plutonium produced could be successfully extracted from the irradiated uranium. Using lanthanum fluoride as a carrier, Seaborg isolated a weighable sample of plutonium in Aug. 1942. At the same time, Isadore Perlman and William J. Knox explored the peroxide method of separation; John E. Willard studied various materials to determine which best adsorbed plutonium; Theodore T. Magel and Daniel K. Koshland, Jr., researched solvent-extraction processes; and Harrison Scott Brown and Orville F. Hill performed experiments into volatility reactions.

Basic research on plutonium’s chemistry continued as did work on radiation and fission products. Seaborg’s discovery and subsequent isolation of plutonium were major events in the history of chemistry, but, like Fermi’s achievement, it remained to be seen whether they could be translated into a production process useful to the bomb effort. In fact, Seaborg’s challenge seemed even more daunting, for while piles had to be scaled up ten to twenty times, a separation plant for plutonium would involve a scale-up of the laboratory experiment on the order of a billion-fold.

PUREX is a chemical method used to purify fuel for nuclear reactors or nuclear weapons. It is an acronym standing for Plutonium Uranium Redox EXtraction. PUREX is the de facto standard aqueous nuclear reprocessing method for the recovery of uranium and plutonium from used ("spent", or "depleted") nuclear fuel. It is based on liquid–liquid extraction ion-exchange.

The PUREX process was invented by Herbert H. Anderson and Larned Brown Asprey at the Metallurgical Laboratory at the University of Chicago, as part of the Manhattan Project under Seaborg; their patent "Solvent Extraction Process for Plutonium" filed in 1947, mentions tributyl phosphate as the major reactant which accomplishes the bulk of the chemical extraction.

What we see above is a later patient for a method that successively comprises: a) co-extracting the uranium (VI), plutonium (IV) and other actinides (IV) or (VI) from an aqueous nitric solution by using solvent phase and scrubbing the latter; b) back-extracting the plutonium in oxidation state (III) from the solvent phase by using an aqueous nitric solution; c) back-extracting the uranium in oxidation state (VI) from the solvent phase by using an aqueous nitric solution; d) concentrating the aqueous nitric solution resulting from step c) with respect to uranium (VI); and it is characterised in that some of the uranium (VI)-concentrated aqueous solution obtained in step d) is used for back-extracting the actinide (IV) or actinides (IV) from the solvent phase during step b) or between steps b) and c).

Collaboration with Du Pont’s Charles Milton Cooper and his staff on plutonium separation facilities began even before Seaborg succeeded in isolating a sample of plutonium. Seaborg was reluctant to drop any of the approaches then under consideration, and Cooper agreed. The two decided to pursue all four methods of plutonium separation but put first priority on the lanthanum fluoride process Seaborg had already developed. Cooper’s staff ran into problems with the lanthanum fluoride method in late 1942, but by then Seaborg had become interested in phosphate carriers. Work led by Stanley G. Thompson found that bismuth phosphate retained over ninety-eight percent plutonium in a precipitate. With bismuth phosphate as a backup for the lanthanum fluoride, Cooper moved ahead on a semi-works near Stagg Field.

Compton’s original plans to build the experimental pile and chemical separation plant on the University of Chicago campus changed during fall 1942. It was decided that it would be safer to put Fermi’s pile in Argonne and build the pilot plant and separation facilities in Oak Ridge than to place these experiments in a populous area. On Oct. 3, 1942, Du Pont agreed to design and build the chemical separation plant. Groves tried to entice further Du Pont participation at Oak Ridge by having the firm prepare an appraisal of the pile project and by placing three Du Pont staff members on the Lewis committee. Because Du Pont was sensitive about its public image (the company was still smarting from charges that it profiteered during WW I), Groves ultimately obtained the services of the giant chemical company for the sum of one dollar over actual costs. In addition, Du Pont vowed to stay out of the bomb business after the war and offered all patents to the United States government.

Groves had done well in convincing Du Pont to join the Manhattan Project. Du Pont’s proven administrative structure assured excellent coordination (Crawford Greenewalt was given the responsibility of coordinating Du Pont and Met Lab planning), and Groves and Compton welcomed the company’s demand that it be put in full charge of the Oak Ridge plutonium project. Du Pont had a strong organisation and had studied every aspect of the Met Lab’s program thoroughly before accepting the assignment. While deeply involved in the overall war effort, Du Pont expected to be able to divert personnel and other resources from explosives work in time to throw its full weight into the Oak Ridge project.

Hanford Engineering Works 

Moving the pilot plutonium plant to Oak Ridge left too little room for the full-scale production plant at the X-10 site and also left too little generating power for yet another major facility. Furthermore, the site was uncomfortably close to Knoxville should a catastrophe occur. Thus the search for an alternate location for the full-scale plutonium facility began soon after Du Pont joined the production team. Compton’s scientists needed an area of approximately 225 square miles. Three or four piles and one or two chemical separation complexes would be at least a mile apart for security purposes, while nothing would be allowed within four miles of the separation complexes for fear of radioactive accidents.

Dec. 16, 1942, found Colonel Franklin T. Matthias of Groves’s staff and two Du Pont engineers headed for the Pacific Northwest and southern California to investigate possible production sites. Of the possible sites available, none had a better combination of isolation, long construction season, and abundant water for hydroelectric power than those found along the Columbia and Colorado Rivers. After viewing six locations in Washington, Oregon, and California, the group agreed that the area around Hanford, Washington, best met the criteria established by the Met Lab scientists and Du Pont engineers. The Grand Coulee and Bonneville Dams offered substantial hydroelectric power, while the flat but rocky terrain provided excellent support for the huge plutonium production buildings. The ample site of nearly one-half million acres was far enough inland to meet security requirements, while existing transportation facilities could quickly be improved and labour was readily available. Pleased with the committee’s unanimous report, Groves accepted its recommendation and authorized the establishment of the Hanford Engineer Works, codenamed Site W

Aerial View of Hanford Community

Not only did they need considerable space for 6 atomic piles, 3 separation plants, production facilities, laboratories, and workers village, but they needed 100,000 kilowatts of continuous power and 25,000 gallons of (soft) water per minute for cooling. Finally the estimated cost for acquiring the entire site (670 square miles) was slightly over $5 million.  

The X-10 complex at Oak Ridge

The fall 1942 planning sessions at the Met Lab led to the decision to  build a  second Fermi pile at Argonne as soon as his experiments on the first were completed and to  proceed on the design of the “Mae West” helium-cooled unit. When Du Pont engineers assessed the Met Lab’s plans in the late fall, they agreed that helium should be given first priority. They placed heavy water second and urged an all out effort to produce more of this highly effective moderator. Bismuth and water were ranked third and fourth in Du Pont’s analysis. 

Priorities changed when Fermi’s calculations demonstrated a higher value for k than anyone had anticipated. Met Lab scientists concluded that a water-cooled pile was now feasible, while Du Pont shifted its interest to air cooling. Since a helium-cooled unit shared important design characteristics with an air-cooled one, Greenewalt thought that an air-cooled semi-works at Oak Ridge would contribute significantly to designing the full-scale facilities at Hanford. Du Pont established the general specifications for the air-cooled semi-works and chemical separation facilities in early 1943. A massive graphite block, protected by several feet of concrete, would contain hundreds of horizontal channels filled with uranium slugs surrounded by cooling air. New slugs would be pushed into the channels on the face of the pile, forcing irradiated ones at the rear to fall into an underwater bucket. The buckets of irradiated slugs would be left to decay for several weeks, then be moved by an underground canal into the chemical separation facility where the plutonium would be extracted with remote control equipment. 

The British designed Magnox, first built at Calder Hall, was built principally to produce plutonium for nuclear weapons. They were pressurised, carbon dioxide cooled, graphite moderated reactors using natural uranium (i.e. unenriched) as fuel and magnox (magnesium-aluminum) alloy as fuel cladding. 

Met Lab activities focused on designing a water-cooled pile for the full-scale plutonium plant. Taking their cue from the Du Pont engineers, who utilized a horizontal design for the air-cooled semi-works, Met Lab scientists abandoned the vertical arrangement with water tanks, which had posed serious engineering difficulties. Instead they proposed to use uranium slugs sealed in aluminum cans inside aluminum tubes. The tubes, laid horizontally through a graphite block, would cool the pile with water injected into each tube. The pile, containing 200 tons of uranium and 1,200 tons of graphite, would need 75,000 gallons of water per minute for cooling.

Greenewalt’s initial response to the water-cooled design was guarded. He worried about pressure problems that might lead to boiling water in individual tubes, corrosion of slugs and tubes, and the 1% margin of safety. But he was even more worried about the proposed helium-cooled model. He feared that the compressors would not be ready in time for Hanford, that the shell could not be made vacuum-tight, and that the pile would be extremely difficult to operate. Du Pont engineers conceded that Greenewalt’s fears were well grounded. Late in Feb. 1943, Greenewalt reluctantly concluded that the Met Lab’s model, while it had its problems, was superior to Du Pont’s own helium cooled design and decided to adopt the water-cooled approach. The Met Lab’s victory in the pile design competition came as its status within the Manhattan Project was changing. Still an exciting place intellectually, the Met Lab occupied a less central place in the bomb project, as Oak Ridge and Hanford rose to prominence. Fermi continued to work on the Stagg Field pile (CP-1), hoping to determine the exact value of k. Subsequent experiments at the Argonne site using CP-2, built with material from CP-1, focussed on neutron capture probabilities, control systems and instrument reliability. Once the production facilities at Oak Ridge and Hanford were underway, however, Met Lab research became increasingly unimportant in the race for the bomb and the scientists found themselves serving primarily as consultants for Du Pont.

While the Met Lab physicists chafed under Du Pont domination, a smoother and quieter relationship existed between the chemists and Du Pont. Seaborg and Cooper continued to work well together, and enough progress was made in the semi-works for the lanthanum fluoride process in late 1942 that Du Pont moved into the plant design stage and converted the semi-works for the bismuth phosphate method. Du Pont pressed for a decision on plutonium extraction methods in late May, 1992. Greenewalt chose bismuth phosphate, though even Seaborg admitted he could find little to distinguish between the two. Greenewalt based his decision on the corrosiveness of lanthanum fluoride and on Seaborg’s guarantee that he could extract at least 50% of the plutonium using bismuth phosphate. Du Pont began constructing the chemical separation pilot plant at Oak Ridge, while Seaborg continued refining the bismuth phosphate method.

It was now Cooper’s job to design the pile as well as the plutonium extraction facilities at Clinton, both complicated engineering tasks made even more difficult by high levels of radiation produced by the process. Not only did Cooper have to oversee the design and fabrication of parts for yet another new Manhattan Project technology, he had to do so with an eye toward planning the Hanford facility. Safety was a major consideration because of the hazards of working with plutonium, which was highly radioactive. Uranium, a much less active element than plutonium, posed far fewer safety problems. 

In July 1942 Compton also setup a health division at the Met Lab and put Robert S. Stone in charge. Stone established emission standards and conducted experiments on radiation hazards, providing valuable planning information for the Oak Ridge and Hanford facilities.

Du Pont broke ground for the X-10 complex at Oak Ridge in Feb. 1943. The site would include an air-cooled experimental pile, a pilot chemical separation plant, and support facilities. Cooper produced blueprints for the chemical separation plants in time for construction to begin in March 1943. A series of huge underground concrete cells, the first of which sat under the pile, extended to one story above ground. Aluminum cans containing uranium slugs would drop into the first cell of the chemical separation facility and dissolve and then go through the extraction process. The pile building went up during the spring and summer, a huge concrete shell seven feet thick with hundreds of holes for uranium slug placement. Slugs were to plutonium piles what the barrier was to gaseous diffusion that is, an obstacle that could shut down the entire process. The Aluminum Company of American (Alcoa) was the only firm left working on a process to enclose uranium-235 in aluminum sheaths, and it was still having problems. Initial production provided mixed results, with many cans failing vacuum tests because of faulty welds.

The moment everyone had been waiting for came in late Oct. 1943, when Du Pont completed construction and tests of the X-10 pile at Clinton Engineer Works. After thousands of slugs were loaded, the pile went critical in the early morning of No. 4, 1943 and produced plutonium by the end of the month. Criticality was achieved with only half of the channels filled with uranium. During the next several months, Compton gradually raised the power level of the pile and increased its plutonium yield.

Chemical separation techniques using the bismuth phosphate process were so successful that Los Alamos received plutonium samples beginning in the spring 1944. Fission studies of these samples at Los Alamos during summer 1944 heavily influenced bomb design.

Back at Hanford

Colonel Matthias returned to the Hanford area to set up a temporary office on Feb. 22, 1943. His orders had been to purchase half a million acres in and around the Hanford-Pasco-White Bluffs area, a sparsely populated region where sheep ranching and farming were the main economic activities. Many of the area’s landowners rejected initial offers on their land and took the Army to court seeking more acceptable appraisals. Matthias adopted a strategy of settling out of court to save time, time being a more important commodity than money to the Manhattan Project.

Matthias received his assignment in late March, 1943. The three water-cooled piles, designated by the letters B, D, and F, would be built about six miles apart on the South bank of the Columbia River. The four chemical separation plants, built in pairs, would be nearly ten miles south of the piles, while a facility to produce slugs and perform tests would be approximately twenty miles southeast of the separation plants near Richland. Temporary quarters for construction workers would be put up in Hanford, while permanent facilities for other personnel would be located down the road in Richland, safely removed from the production and separation plants.

During summer 1943, Hanford became the Manhattan Project’s newest atomic boomtown. Thousands of workers poured into the town, many of them to leave in discontent. Well situated from a logistical point of view, Hanford was a sea of tents and barracks where workers had little to do and nowhere to go. Du Pont and the Army coordinated efforts to recruit labourers from all over the country for Hanford, but even with a relative labour surplus in the Pacific Northwest, shortages plagued the project. Conditions improved significantly during the second half of the year, with the addition of recreational facilities, higher pay, and better overall service for Hanford’s population, which reached 50,000 by summer 1944. Hanford still resembled the frontier and mining towns once common in the west, but the rate of worker turnover dropped substantially.

Groundbreaking for the water-cooling plant for the B pile (above is the B reactor face), the western most of the three, took place on Aug. 27, 1944, less than two weeks before Italy’s surrender to the Allies on Sept. 8, 1944. Work on the pile itself began in Feb. 1945, with the base and shield being completed by mid-May 1945. It took another month to place the graphite pile and install the top shield and two more months to wire and pipe the pile and connect it to the various monitoring and control devices.

We can see above T plant being built. T and U plants were what might be called first generation chemical separation facilities, and REDOX can be called a second generation process, i.e. the PUREX process. 

At Hanford, irradiated uranium slugs would drop into water pools behind the piles and then be moved by remote controlled rail cars to a storage facility five miles away for transportation to their final destination at one of the two chemical separation locations, designated 200-West and 200-East. The T and U plants were located at 200-West, while a single plant, the B unit, made up the 200-East complex (the planned fourth chemical separation plant was not built). The Hanford chemical separation facilities were massive scaled-up versions of those at Oak Ridge, each containing separation and concentration buildings in addition to ventilation (to eliminate radioactive and poisonous gases) and waste

storage areas. Labour shortages and the lack of firm blueprints forced Du Pont to stop work on the 200 areas in summer 1943 and concentrate its forces on B pile, with the result that 1943 construction progress on chemical separation was limited to digging two huge holes in the ground.

Here we have the “Queen Mary” structure going up, and one of the finished cells. This was T-plant, and it got its name because it was long and thin like the ocean liner. Today T-plant is no longer operational, and it is now used as a decommissioning and repair facility for treating, verifying and re-packaging waste. T Plant is the only processing canyon at Hanford that remains in operation, although its mission today does not have anything to do with producing plutonium for weapons. The B Plant, S Plant (REDOX), U Plant, and PUREX were the other four processing canyons at Hanford which have long been shut down.


Both 221-T and 221-U, the chemical separation buildings in the 200-West complex, were finished by Dec. 1944. 221-B, their counterpart in 200-East, was completed in spring 1945. Nicknamed Queen Mary’s by the workers who built them, the separation buildings were awesome canyon-like structures 800 feet long, 65 feet wide, and 80 feet high containing forty process pools. The interior had an eerie quality as operators behind thick concrete shielding manipulated remote control equipment by looking through television monitors and periscopes from an upper gallery. Even with massive concrete lids on the process pools, precautions against radiation exposure were necessary and influenced all aspects of plant design.

Construction of the chemical concentration buildings (224-T, -U, and -B) was a less daunting task because relatively little radioactivity was involved, and the work was not started until very late 1944. The 200-West units were finished in early Oct. 1944, the East unit in Feb. 1945. In the Queen Mary’s, bismuth phosphate carried the plutonium through the long succession of process pooIs. The concentration stage was designed to separate the two chemicals. The normal relationship between pilot plant and production plant became evident when the Oak Ridge pilot plant reported that bismuth phosphate was not suitable for the concentration process but that Seaborg’s original choice, lanthanum fluoride, worked quite well. Hanford, accordingly incorporated this suggestion into the concentration facilities. The final step in plutonium extraction was isolation, performed in a more typical laboratory setting with little radiation present. Here Perlman’s earlier research on the peroxide method paid off and was applied to produce pure plutonium nitrate. The nitrate was converted to metal in Los Alamos, New Mexico.

Los Alamos - Project Y

The final link in the Manhattan Project’s far flung network was the Los Alamos Scientific Laboratory in Los Alamos, New Mexico. The laboratory that designed and fabricated the first atomic bombs, code named Project Y, began to take shape in spring 1942 when Conant suggested to Bush that the Office of Scientific and Research Development and the Army form a committee to study bomb development. Bush agreed and forwarded the recommendation to Vice President Wallace, Secretary of War Stimson, and General Marshall (the Top Policy Group). By the time of his appointment in late Sept. 1942, Groves had orders to setup a committee to study military applications of the bomb. Meanwhile, sentiment was growing among the Manhattan scientists that research on the bomb project needed to be better coordinated. Oppenheimer, among others, advocated a central facility where theoretical and experimental work could be conducted according to standard scientific protocols. This would insure accuracy and speed up progress. Oppenheimer suggested that the bomb laboratory operate secretly in an isolated area but allow free exchange of ideas among the scientists on the staff. Groves accepted Oppenheimer’s suggestion and began seeking an appropriate location.

It has been suggested that Groves wanted an American Nobel Prize winner as director of Los Alamos, but Lawrence, Rabi, and Compton were all already involved in critical projects, so Groves turned to Oppenheimer because he had been impressed by the physicist during a meeting on fission research. 

The search for a bomb laboratory site quickly narrowed to two places in northern New Mexico, Jemez Springs and the Los Alamos Boys Ranch School, locations Oppenheimer knew well since he had a ranch nearby in the Pecos Valley of the Sangre de Cristo Mountains. In mid-November, Oppenheimer, Groves, Edwin Mattison McMillan, and Lieutenant Colonel W. H. Dudley visited the two sites and chose Los Alamos. Located on a mesa about thirty miles northwest of Santa Fe, Los Alamos was virtually inaccessible. It would have to be provided with better water and power facilities, but the laboratory community was not expected to be very large. The boys’ school occupying the site was eager to sell, and Groves was equally eager to buy. By the end of 1942 the district engineer in  Albuquerque had orders to begin construction, and the University of California had contracted to provide supplies and personnel.

The new site was initially classed as a demolition range, then as Site Y, Project Y, Zia Project, Santa Fe, or finally just Los Alamos. The site was officially activated as a military establishment on April 1, 1943, and was unique in that it was a separate organisation directly responsible to Groves, and run by the University of California. 

Oppenheimer, as head of the new laboratory, proved to be an excellent director despite initial concerns about his administrative inexperience, leftist political sympathies, and lack of a Nobel Prize when several scientists he would be directing were prizewinners. Groves worked well with Oppenheimer although the two were fundamentally different in temperament. Groves was a practical-minded military man, brusque and goal oriented. His aide, Colonel Nichols, characterised his heavyset boss as ruthless, egotistical, and confident, “the biggest S.O.B. I have ever worked for. He is most demanding. He is most critical. He is always a driver, never a praiser. He is abrasive and sarcastic”, Nichols admitted, however, that if he had it to do over again, he would once again “pick General Groves [as his boss]” because of his unquestioned ability. Groves demanded that the Manhattan Project scientists spend all their time on the bomb and resist the temptation, harmless enough in peacetime, to follow lines of research that had no direct applicability to immediate problems.

In contrast to Groves, Oppenheimer was a philosophical man, attracted to Eastern mysticism and of a decidedly theoretical inclination and sensitive nature. A chain-smoker given to long working hours, Oppenheimer appeared almost emaciated. The Groves-Oppenheimer alliance, though not one of intimacy, was marked by mutual respect and was a major factor in the success of the Manhattan Project.

Oppenheimer insisted, with some success, that scientists at Los Alamos remain as much an academic community as possible, and he proved adept at satisfying the emotional and intellectual needs of his highly distinguished staff. Hans Bethe, head of the theoretical division, remembered that nobody else in that laboratory “... even came close to him. In his knowledge. There was human warmth as well. Everybody certainly had the impression that Oppenheimer cared what each particular person was doing. In talking to someone he made it clear that that person’s work was important for the success of the whole project”.

Oppenheimer had a chance to display his persuasive abilities early when he had to convince scientists, many of them already deeply involved in war related research in university laboratories, to join his new organisation.

Complicating his task was the fact that Groves and Oppenheimer planned to operate Los Alamos as a military laboratory. Oppenheimer accepted Groves’s rationale for this arrangement but soon found that scientists objected to working as commissioned officers and feared that the military chain of command was ill suited to scientific decision making. The issue came to a head when Oppenheimer tried to convince Robert F. Bather and Isidor I. Rabi of the Massachusetts Institute of Technology’s Radiation Laboratory to join the Los Alamos team. Neither thought a military environment was conducive to scientific research. At Oppenheimer’s request, Conant and Groves wrote a letter explaining that the secret weapon-related research had presidential authority and was of the utmost national importance.

The letter promised that the laboratory would remain civilian through 1943, when it was believed that the requirements of security would require militarisation of the final stages of the project (in fact, militarization never took place). Oppenheimer would supervise all scientific work, and the military would maintain the post and provide security.

Oppenheimer spent the first three months of 1943 tirelessly crisscrossing the U.S. in an attempt to put together a first-rate staff, an effort that proved highly successful. Even Bather signed on, though he promised to resign the moment militarisation occurred. Rabi, though he did not move to Los Alamos, became a valuable consultant. As soon as Oppenheimer arrived at Los Alamos in mid-March, recruits began arriving from universities across the United States, including California, Minnesota, Chicago, Princeton, Stanford, Purdue, Columbia, IowaState, and the Massachusetts Institute of Technology, while still others came from the Met Lab and the National Bureau of Standards. Virtually overnight Los Alamos became an ivory tower frontier boomtown, as scientists and their families, along with nuclear physics equipment, including two Van de Graaff’s, a Cockroft-Walton accelerator, and a cyclotron, arrived caravan fashion at the Santa Fe railroad station and then made their way up to the mesa along the single primitive road. It was a most remarkable collection of talent and machinery that settled this remote outpost of the Manhattan Project.

It has been noted that the average age of the scientists in Los Alamos in 1945 was 29 years old.

Theory and the “Gadget”

Organised by Oppenheimer into specialized research and technical divisions and groups, the Los Alamos scientists divided their efforts between two fundamental tasks: solving the theoretical and experimental problems of a fission bomb, and working out the complex ordnance and engineering problems of weapon design and fabrication. Their concentrated activity over a two-year period, from 1943 to 1945, transformed the laboratory, for all intents and purposes, into a weapon assembly and test plant.

“Gadget” was the code name given to the first bomb tested as part of the Trinity nuclear test.

The initial spartan environment of “the Hill” (which included box lunches and temporary housing) was without doubt quite a contrast to the comfortable campus settings so familiar to many of the staff. But the laboratory’s work began even as the Corps of Engineers struggled to provide the amenities of civilized life. The properties of uranium were reasonably well understood, those of plutonium less so, and knowledge of fission explosions entirely theoretical. That 2.2 secondary neutrons were produced when uranium-235 fissioned was accepted, but while Seaborg’s team had proven in March 1941 that plutonium underwent neutron induced fission, it was not known yet if plutonium released secondary neutrons during bombardment.

A plutonium pit is the key part of a nuclear warhead. Each pit, slightly warm to the touch, has about 30 parts, which are often coated with nickel or beryllium. Engineered to extraordinary tolerances, the parts fit together like a three-dimensional puzzle. The U.S. lost the capability to produce plutonium pits for its nuclear weapons stockpile in 1989 after a raid by the Federal Bureau of Investigation investigating alleged environmental crimes at the Rocky Flats Plant near Denver, Colorado. At that time, the Rocky Flats Plant was the stockpile plutonium pit production facility for the American nuclear weapons complex. In 1996, DOE officially decided to relocate plutonium pit production to the Los Alamos National Laboratory.

Since then, the U.S. has made at most 11 pits per year. U.S. policy is to maintain existing nuclear weapons. To do this, the Department of Defense (DoD) states that it needs the Department of Energy (DoE), which maintains U.S. nuclear weapons, to produce 50-80 pits per year by 2030. We have to remember that the the Pantex nuclear facility in Amarillo, Texas stores a staggering 15,000 plutonium pits. Meanwhile, Los Alamos planned the construction of the Chemistry and Metallurgy Research Replacement (CMRR), to carry out design and support-work for the pits (replacing the old CMR facility). Wikipedia tells us that reconstruction was in 3 phases, for a total cost of $4-5 billion. Wikipedia also tells us that the first phase was delayed in 2012, other reports say it was cancelled in 2014. The plans changed to the building of a upgraded facility and underground modular plutonium pit facility for $4.3 billion (recently proposed to defer for 5 years). One part of the original plans, the Radiological Laboratory, Utility, and Office Building (RLUOB) was completed, and began radiological operations in Aug. 2014. U.S. Congressional oversight has called Los Alamos plans a “largely-uncoordinated pastiche of plutonium-related line-item construction projects”, with inappropriate “mission needs”, no functional parameters, and major uncertainties about schedule, cost, scope, and overall feasibility. 

Critics argue that the new facility is unnecessary for maintaining the nuclear stockpile and may be seen as a threatening development because it allows for the production of new types of nuclear weapons. Opponents to these new developments claim that there are 2,070 deployed weapons, plus 2,600 spare and reserve weapons, plus 2,300 warheads not being maintained, plus about 16,00 surplus pit, of which 5,000 are in strategic reserve. 

The theoretical consensus was that chain reactions took place with sufficient speed to produce powerful releases of energy and not simply explosions of the critical mass itself, but only experiments could test the theory. The optimum size of the critical mass remained to be established, as did the optimum shape. When enough data were gathered to establish optimum critical mass, optimum effective mass still had to be determined. That is, it was not enough simply to start a chain reaction in a critical mass, it was necessary to start one in a mass that would release the greatest possible amount of energy before it was destroyed in the explosion.

In addition to calculations on uranium and plutonium fission, chain reactions, and critical and effective masses, work needed to be done on the ordnance aspects of the bomb, or “gadget” as it came to be known. Two subcritical masses of fissionable material would have to come together to form a supercritical mass for an explosion to occur.

Furthermore, they had to come together in a precise manner and at high speed. Measures also had to be taken to insure that the highly unstable subcritical masses did not pre-detonate because of spontaneously emitted neutrons or neutrons produced by alpha particles reacting with lightweight impurities. The chances of pre-detonation could be reduced by purification of the fissionable material and by using a high-speed firing system capable of achieving velocities of 3,000 feet per second. A conventional artillery method, of firing one subcritical mass into the other was under consideration for uranium-235, but this method would work for plutonium only if absolute purification of plutonium could be achieved.

Bomb designers, unable to solve the purification problem, turned to the relatively unknown implosion method for plutonium. With implosion, symmetrical shockwaves directed inward would compress a subcritical mass of plutonium packed in a nickel casing (tamper), releasing neutrons and causing a chain reaction.

Always in the background loomed the hydrogen bomb, a thermonuclear device considerably more powerful than either a uranium or plutonium device but one that needed a nuclear fission bomb as a detonator. Research on the hydrogen bomb, or “Super”, was always a distant second in priority at Los Alamos, but Oppenheimer concluded that it was too important to ignore. After considerable thought, he gave Teller permission to devote himself to the “Super”. To make up for Teller’s absence, Rudolf Peierls, one of a group of British scientists who reinforced the Los Alamos staff at the beginning of 1944, was added to Bethe’s theory group in mid-1944. Another member of the British contingent was the Soviet agent Klaus Fuchs, who had been passing nuclear information to the Russians since

1942 and continued doing so until 1949 when he was caught and convicted of espionage (and subsequently exchanged).

The first few months at Los Alamos were occupied with briefings on nuclear physics for the technical staff and with planning research priorities and organizing the laboratory. Groves called once again on Warren Lewis to head a committee, this time to evaluate the Los Alamos program. The committee’s recommendations resulted in the coordinated effort envisioned by those who advocated a unified laboratory for bomb research. Fermi took control of critical mass experiments and standardization of measurement techniques. Plutonium purification work, begun at the Met Lab, became high priority at Los Alamos, and increased attention was paid to metallurgy. The committee also recommended that an engineering division be organized to collaborate with physicists on bomb design and fabrication. The laboratory was thus organized into four divisions: theoretical (Hans A. Bethe); experimental physics (Robert F. Bather); chemistry and metallurgy (Joseph William Kennedy); and ordnance (Navy Captain William S. “Deke” Parsons). Like other Manhattan Project installations, Los Alamos soon began to expand beyond initial expectations. As director, Oppenheimer shouldered burdens both large and small, including numerous mundane matters such as living quarters, mail censorship, salaries, promotions, and other “quality of life” issues inevitable in an intellectual pressure-cooker with few social amenities. Oppenheimer relied on a group of advisers to help him keep the “big picture” in focus, while a committee made up of Los Alamos group leaders provided day-today communications between divisions.

Los Alamos makes progress

Early experiments on both uranium and plutonium provided welcome results. Uranium emitted neutrons in less than a billionth of a second, i.e. just enough time, in the world of nuclear physics, for an efficient explosion. Ernilio Segrè later provided an additional cushion with his discovery in Dec. 1943 that, if cosmic rays were eliminated, the subcritical uranium masses would not have to be brought together as quickly as previously thought, nor would the uranium have to be as pure. Muzzle velocity for the scaled down artillery piece could be lower, and a gun-type weapon could be shorter and lighter.

Segrè’s tests on the first samples of plutonium demonstrated that plutonium emitted even more neutrons than uranium due to the spontaneous fission of plutonium-240. Both theory and experimental data now agreed that a bomb using either element would detonate if it could be designed and fabricated into the correct size and shape. But many details remained to be worked out, including calculations to determine how much uranium-235 or plutonium would be needed for an explosive device.

So the physicists gathered considerable data on the effect of cosmic rays on fissioning, on measurement of nuclear cross sections, on scattering phenomena, and on other aspects of the fission process that related to bomb specifications and efficiency. With this data they were able to calculate by the summer of 1944 that the destructive effect of either an implosion- or gun-type bomb would justify the effort required to fabricate it.

Bather’s engineering division patiently generated the essential cross-sectional measurements needed to calculate critical and efficient mass. (The cross section is a measurement that indicates the probability of a nuclear reaction taking place). The same group utilized particle accelerators to produce the large numbers of neutrons needed for its cross-sectional experiments. Bather’s group also compiled data that helped identify tamper materials that would most effectively push neutrons back to the core and enhance the efficiency of the explosion. Despite Los Alamos’s postwar reputation as a mysterious retreat where brilliant scientists performed miracles of nuclear physics, much of the work that led to the atomic bombs was extremely tedious.

A “tamper” is an optional layer of dense material surrounding the fissile material. Due to its inertia it delays the expansion of the reacting material, increasing the efficiency of the weapon. Often the same layer serves both as tamper and as neutron reflector.

The chemists’ job was to purify the uranium-235 and plutonium, reduce them to metals, and process the tamper material. Only highly purified uranium and plutonium would be safe from pre-detonation. Fortunately purification standards for uranium were relatively modest, and the chemical division was able to focus its effort on the lesser known plutonium and make substantial progress on a multi-step precipitation process by summer 1944. The metallurgy division had to turn the purified uranium-235 and plutonium into metal. Here, too, significant progress was made by summer of 1944, as the metallurgists adapted a stationary-bomb technique initially developed at Iowa State University. Parsons, in charge of ordnance engineering, directed his staff to design two artillery pieces of relatively standard specifications except for their extremely light barrels. One for a uranium weapon and one for a plutonium bomb. The weapons needed to achieve high velocities, but they would not have to be durable since they would only be fired once. Here again early efforts centered on the more problematic plutonium weapon, which required a higher velocity due to its higher risk of pre-detonation. Two plutonium guns arrived in March 1944 and were field-tested successfully. In the same month, two uranium guns were ordered.

Since no one could yet answer the question: How much fissionable material would be needed for an effective weapon? One way to increase the efficiency of a fission bomb was to achieve maximum purity in the active materials. Hence, a major program of the laboratory's chemistry and metallurgy division was to improve the methods for purifying uranium-235 and plutonium-239. Because purity requirements for uranium were about one-third less than those for plutonium and because, until early 1944, there was not enough plutonium-239 available to permit effective work on its purification, the chemists experimented with uranium but with the purpose of developing techniques that might also be used with plutonium. When sufficient amounts of plutonium-239 arrived from the Clinton pile, the chemists developed both wet and dry purification processes. Subsequently, they employed the more satisfactory wet process in the final purification of most the plutonium for the bomb.

Before uranium-235 or plutonium-239 could be used in a fission bomb, they had to be converted into metal of the proper configuration and purity. Metallurgists at Los Alamos faced a number of problems in making uranium or plutonium metal of the desired quality, including the tendency of uranium to catch fire during processing and the difficulty of handling the highly reactive and poisonous plutonium. For forming uranium into metal, they experimented with electrolytic and centrifuge processes but finally settled upon a modification of the stationary bomb method, devised earlier at Iowa State. 

For plutonium, the metallurgists were as handicapped as the chemists, with only microscopic quantities available. Fortunately, many of the methods they developed for uranium proved adaptable to plutonium. Again like the chemists, the metallurgists had to devote considerable effort to devising improved recovery methods so that virtually none of the precious metal would be lost in processing it for use in a weapon.

Early Implosion Work

Parsons assigned implosion studies a low priority and placed the emphasis on the more familiar artillery (gun) method. Consequently, Seth Henry Neddermeyer performed his early implosion tests in relative obscurity. Neddermeyer found it difficult to achieve symmetrical implosions at the low velocities he had achieved. When the Princeton mathematician John von Neumann, a Hungarian refugee, visited Los Alamos late in 1943, he suggested that high-speed assembly and high velocities would prevent pre-detonation and achieve more symmetrical explosions. A relatively small, subcritical mass could be placed under so much pressure by a symmetrical implosion that an efficient detonation would occur.

Less critical material would now be required, bombs could be ready earlier, and extreme purification of plutonium would be unnecessary. Von Neumann’s theories excited Oppenheimer, who assigned Parsons’s deputy, George B. Kistiakowsky, the task of perfecting implosion techniques. Because Parsons and Neddemeyer did not get along, it was Kistiakowsky who worked with the scientists on the implosion project. While experiments on implosion and explosion continued, Parsons directed much of his effort toward developing bomb hardware, including arming and wiring mechanisms and fusing devices. Working with the Army Air Force, Parsons’s group developed two bomb models by March 1944 and began testing them with B-29’s. “Thin Man”, named for President Roosevelt, utilized the plutonium gun design, while “Fat Man”, named after Winston Churchill, was an implosion prototype. (Segrè’s lighter, smaller uranium gadget became “Little Boy”, “Thin Man’s” younger brother). 

It all dates back to a meeting in April 1943, where the gun approach appeared to provide the surest path to an atomic bomb. It was for this reason that Groves recruited Parsons to head the gun assembly research and development. It was on July 4, 1943, whilst Parsons was absent, that some of his staff conducted a successful proof-of-principle implosion experiment, thus keeping the implosion option alive.    

Primarily because of the undeveloped state of the art, interest in implosion research for a time ranked second to that in gun assembly research. Since April 1943, physicist Seth H. Neddermeyer from the California Institute of Technology had been conducting laboratory experiments with high explosives, designed to test the feasibility of the implosion principle. Neddermeyer's project had definitely remained a "dark horse" in the race for completion of a workable atomic device.

But all of this changed with the arrival of John von Neumann in midsummer 1943. The widely respected Hungarian-born mathematician from the Institute for Advanced Study at Princeton had been carrying out work on shock waves for the NDRC. Applying knowledge of explosives gained in his work with shaped charges, he theorized the likely effects of increasing the velocity of converging focused active material in the implosion bomb. His calculations convinced him that if the mechanical problems of achieving higher velocity could be solved, an implosion bomb would attain criticality using less active material of a considerably lower level of purity than hitherto believed possible.

If he were correct, implosion offered a means to save precious months in developing a weapon, provided, of course, that ways could be devised to avoid pre-detonation and achieve symmetry in the imploding shock wave inside the bomb.

By early fall 1943 Oppenheimer, Groves, Conant, and the other project leaders were re-evaluating implosion. Groves conferred with George B. Kistiakowsky, the distinguished Harvard chemist who was an expert on explosives, and with Oppenheimer and members of the laboratory's implosion study group. This led to a decision by Oppenheimer and the laboratory's governing board to expand the implosion program immediately, beginning with construction of an on-site plant for casting and trimming test components and installation of the unusual facilities required for testing implosion devices.

In early Nov. 1943, Groves and Conant outlined the advantages of implosion to the Military Policy Committee. The following February, the committee informed the President that "there is a chance, and a fair one, if a process involving the use of a minimum amount of material proves feasible, that the first bomb can be produced in the late fall of 1944".

Elimination of “Thin Man”

Thin Man” was eliminated four months later because of the plutonium-240 contamination problem. Seaborg had warned that when plutonium-239 was irradiated for a length of time it was likely to pick up an additional neutron, transforming it into plutonium-240 and increasing the danger of pre-detonation (the bullet and target in the plutonium weapon would melt before coming together). Measurements taken at Clinton confined the presence of plutonium-240 in the plutonium produced in the experimental pile. On July 17, 1944, the difficult decision was made to cease work on the plutonium gun method. Plutonium could be used only in an implosion device, but in summer 1944 an implosion weapon looked like a long shot.

So in July 1944, Los Alamos scientists discovered the disquieting new data on the plutonium that would later be produced in the Hanford piles. The composition of its neutron background would cause pre-detonation in the plutonium gun. Project scientists had known for some time that in the process of irradiating uranium in the pile some of the plutonium-239 was likely to pick up an extra neutron, forming plutonium-240. When plutonium from the Clinton pilot pile became available in the spring of 1944, the radioactivity group at Los Alamos ran a series of tests that confirmed the presence of plutonium-240 and indicated it would be present in far larger amounts in plutonium from the Hanford piles. Hence, the neutron background of the active material for the bombs would be several hundred times greater than was permissible. While the plutonium-240 could be separated from the plutonium-239 by the electromagnetic process, construction of a plant to do so would delay production of a plutonium weapon for many months.

"Thin Man" plutonium gun test casings at Wendover Army Air Field, as part of Project Alberta in the Manhattan Project. "Fat Man" casings can be seen behind them.s

One report mentions that in spring 1940, an experiment conducted by a small group of graduate students near the location of Ashley Pond’s long-abandoned Pajarito Club, discovered an isotopic impurity in plutonium, one that could not be removed. If plutonium was used in a gun assembled weapon, this impurity would cause a premature explosion,  spontaneous fission would cause a pre-detonation. Without an alternative assembly method, plutonium could not be used at all for an atomic bomb.

Ashley Pond was a Detroit businessman who took an option out on the land in 1913, in order to build a private club (resort or “dude” ranch) for the wealthy. It was called The Pajarito Club after the Pajarito plateau, but it had already failed by 1916. Ashley Pond went on to found of the Los Alamos Ranch School in 1917. 

Oppenheimer informed Conant of the plutonium-240 problem in early July 1944, and it was decided that the pre-detonation threat posed by plutonium-240 made the use of plutonium in the gun-type bomb impracticable and work on this system should be suspended immediately. With this decision, even greater urgency was placed on the development of a workable implosion weapon, in which the plutonium-240, because of the higher velocities involved, would be unlikely to cause pre-detonation.

Abandonment of the plutonium gun project eliminated a shortcut to the bomb. This necessitated a revision of the estimates of weapon delivery Bush had given the President in 1943. The new timetable, presented to General Marshall by Groves on Aug. 7, 1944, two months after the Allied invasion of France began at Normandy on June 6, 1944, promised small implosion weapons of uranium or plutonium in the second quarter of 1945 if experiments proved satisfactory. More certain was the delivery of a uranium gun bomb by Aug. 1, 1945, and the delivery of one or two more by the end of that year.

Abandonment of the plutonium gun compelled General Groves to revise his predictions on when an atomic weapon would be ready for employment against the enemy. In a progress report to General Marshall in early Aug. 1944, he presented a revised timetable of weapon production: five to eleven implosion bombs in the period from March through June 1945, with an additional twenty to forty implosion bombs of the same size by the end of the year. He cautioned, however, that this schedule would not apply "if experiments yet to be conducted with an implosion type bomb do not fulfill expectations and we are required to rely on the gun type alone" and suggested that, if this delay should occur, the first bomb would not be ready until Aug. 1, 1945, with one or two more by the year's end. In Groves's opinion, any delay virtually guaranteed that the bomb would not be used against Germany, which by the late summer of 1944 appeared likely to be defeated within a few months. And to many, even the bomb's use against Japan seemed doubtful.

Nuclear ordnance

Initially the first priority for the Los Alamos’s ordnance division was design and fabrication of a plutonium-projectile gun. This gun type posed more problems than a uranium gun, because of plutonium-239's higher propensity to pre-detonation, but the division's theory that a gun with sufficient muzzle velocity to avoid pre-detonation with plutonium-239 was certain to be suitable for uranium-235 justified the concentration of effort. Using standard ordnance and interior ballistics data obtained from the National Defense Research Committee (NDRC), the ordnance division had its design engineers complete the drawings for a high-velocity gun and, with subsequent approval from the Navy's Bureau of Ordnance, ordered forgings for two guns from the Naval Gun Factory in Washington, D.C. In the meantime, while the guns were being manufactured, Captain Parsons arranged for construction of the Anchor Ranch Proving Ground, some 8 miles east of the central laboratory facilities, where, by September 1943, the division's proving ground group began testing and perfecting gun performance techniques.

Engineers finally established the exact specifications of a low-velocity gun, to be used with uranium-235. Hence, because these specifications were considerably less stringent than previously anticipated for a uranium-235 gun, the engineers were able to reduce the original muzzle velocity requirements. This achievement made it possible for the division to place in March 1944 an order with the Naval Gun Factory for three of these uranium guns, which was much earlier than expected and just days after the factory had delivered the first two plutonium prototypes to Los Alamos (which later proved to be useless due the contamination problem).

Here we see “Little boy” in the bomb pit on Tinian Island, and being lifted into Enola Gay.  

The gun-type model, the "Thin Man," was about 10 feet in length, with a varying diameter of 1.5 to 2.5 feet, and had an estimated weight (when loaded) of 5 tons. The implosion-type model, the "Fat Man," was almost as long (9 feet) but thicker, tapering down from a hemispherical nose measuring 5 feet in diameter to a tail of about 3 feet, and had an estimated weight (when loaded) of 6 tons. Captain Parsons had models constructed at the Applied Physics Laboratory in Silver Spring, Maryland, and tested at the Naval Proving Ground on the Potomac River at Dahlgren, Virginia. The laboratory's delivery group then conducted in-flight tests in a modified B-29, dropping dummy models of both types of bombs, at the Muroc Army Air Field near San Francisco. The ballistic characteristics of “Thin Man” were satisfactory, but “Fat Man” displayed serious instability, fortunately soon overcome by a relatively simple modification in the tail assembly.

Over time there was an inevitable shift in emphasis from research and experimentation to engineering, fabrication, and testing. Construction crews, under direction of Maj. Wilber A. Stevens, completed a number of essential test areas (eventually there would be more than thirty of these). They had built a facility for casting containers for explosive charges at the Anchor Ranch Proving Ground and, less than a mile to the south, were well advanced on a much larger and more elaborately equipped area, designated S (for Sawmill) Site, with a laboratory, shops, powder magazines, and even a dining hall. In addition, Major Stevens's crews had begun work on several outlying sites required especially for testing various implosion devices. Ordnance teams from Los Alamos also assembled and tested bomb components at test sites at Wendover Field (Utah), Inyokern (California), and Alamogordo Army Air Field (New Mexico). For these tests, the laboratory procured normal weapon components and high explosives from a variety of government and private suppliers, however for special parts and materials that were unobtainable, the laboratory itself had to function as an ordnance manufacturing plant. The best illustration of this effort was the major task of converting uranium-235 and plutonium-239 into metal bomb components.

Japan becomes the target

Marshall and Groves acknowledged that a German surrender might take place by summer 1945, thus making it probable that Japan would be the target of any atomic bombs ready at that time.

It was still unclear if even the Aug. 1, 1945, deadline could be met. While expenditures reached $100 million per month by mid-1944, the Manhattan Project’s goal of producing weapons for the current war was not assured. Operational problems plagued the Y-12 electromagnetic facility just coming on line. The K-25 gaseous diffusion plant threatened to become an expensive white elephant if a suitable barrier could not be fabricated. And the Hanford piles and separation facilities faced an equally serious threat as not enough of the uranium-containing slugs to feed the pile were available. Even assuming that enough uranium or plutonium could be delivered by the production facilities built in such great haste, there was no guarantee that the Los Alamos laboratory would be able to design and fabricate weapons in time. Only the most optimistic in the Manhattan Project would have predicted, as Groves did when he met with Marshall, that a bomb or bombs powerful enough to make a difference in the current war would be ready by Aug. 1, 1945.

During winter 1944-45 there was substantial progress at Oak Ridge, thanks to improved performance in each of the production facilities and Nichols’s work in coordinating a complicated feed schedule that maximized output of enriched uranium by utilizing the electromagnetic, thermal diffusion, and gaseous diffusion processes in tandem. Nine Alpha and three Beta racetracks were operational and, while not producing up to design potential, were becoming significantly more reliable because of maintenance improvements and chemical refinements introduced by Tennessee Eastman. The S-50 thermal diffusion plant being built by the H. K. Ferguson Company was almost complete and a section of S-50 was producing small amounts of enriched material, and the K-25 gaseous diffusion plant, complete with barriers, was undergoing final leak tests. By March 1945, Union Carbide had worked out most of the kinks in K-25 and had started recycling uranium hexafluoride through the system. S-50 was finished at the same time that the Y-12 racetracks were demonstrating increased efficiency. The Beta calutrons at the electromagnetic plant were producing weapon-grade uranium-235 using feed from the modified Alpha racetracks and the small output from the gaseous diffusion and thermal diffusion facilities. Oak Ridge was now sending enough enriched uranium-235 to Los Alamos to meet experimental needs. To increase production, Grove sponsored an additional gaseous diffusion plant (K-27) for low-level enrichment and a fourth Beta track for high-level enrichment, both to be completed by Feb. 1946, in time to contribute to the war against Japan, which many thought would not be conclude before summer 1946.

With the abandonment of the plutonium gun bomb in July 1944, planning at Hanford became more complicated. Pile B was almost complete, as was the first chemical separation plant, while Pile D was at the halfway point. Pile F was not yet under construction. If implosion devices using plutonium could be developed at Los Alamos, the three piles would probably produce enough plutonium for the weapons required, but as yet no one was sure of the amount needed.

Excitement mounted at Hanford as the date for pile start-up approached. Fermi placed the first slug in Pile B on Sept. 13, 1944. Final checks on the pile had been uneventful. The scientists could only hope they were accurate, since once the pile was operational the intense radioactivity would make maintenance of many components impossible. Loading slugs and taking measurements took two weeks. From just after midnight until approximately 3:00 a.m. on Sept. 27, 1944, the pile ran without incident at a power level higher than any previous chain reaction (though only at a fraction of design capacity). The operators were elated, but their excitement turned to astonishment when the power level began falling after three hours. It fell continuously until the pile ceased operating entirely on the evening of Sept. 28, 1944. By the next morning the reaction began again, reached the previous day’s level, then dropped.

Xenon Poisoning

Hanford scientists were at a loss to explain the pile’s failure to maintain a chain reaction (this is now called an iodine “pit”). Only the foresight of Du Pont’s engineers made it possible to resolve the crisis. The cause of the strange phenomenon proved to be xenon poisoning. Xenon, a fission product isotope with a mass of 135, was produced as the pile operated. It captured neutrons faster than the pile could produce them, causing a gradual shutdown. With shutdown, the xenon decayed, neutron flow began, and the pile started up again. Fortuitously, despite the objections of some scientists who complained of Du Pont’s excessive caution, the company had installed a large number of extra tubes. This design feature meant that Pile B could be expanded to reach a power level sufficient to overwhelm the xenon poisoning. Success was achieved when the first irradiated slugs were discharged from Pile B on Christmas Day, 1944. The irradiated slugs, after several weeks of storage, went to the chemical separation and concentration facilities. By the end of Jan. 1945, the highly purified plutonium underwent further concentration in the completed chemical isolation building, where remaining impurities were removed successfully. Los Alamos received its first plutonium on Feb. 2, 1945.

Xenon-135 is a fission product of uranium, and is a “potent” neutron absorber, with a thermal neutron absorption cross section of between 2.6 x 106 barns. So Xenon-135 is about 4000 times better than uranium-235 (cross-section of 400-600 barns for thermal fission) in absorbing thermal neutrons (essential to the chain reaction). About 6.6% of all fissions in uranium-235 end up producing a xenon-135 fission product (along with the other fission product strontium-99 with a very, very short half-life of 0.2 s). However about 95% of xenon-135 actually comes from the β-decay of another fission product, iodine-135 (half-life 6.57 hrs.), and indirectly from tellurium-135 through iodine-135. Xenon-135 is removed from the reactor by neutron absorption (radiative capture to xenon-136) or decay (half-life 9.1hrs.). In some reactor types decay can represent only 10% of xenon-135 loss. 

Samarium-149 is the next most important absorber of thermal neutrons (cross section of 4.1 x 104 barns).

The so-called “xenon transient” when shutting down a reactor was part of the problem in the Chernobyl disaster.  

The Final Push

Oppenheimer acted quickly to maximize the laboratory’s efforts to master implosion. Only if the implosion method could be perfected would the plutonium produced at Hanford come into play. Without either a plutonium gun bomb or implosion weapon, the burden would fall entirely on uranium and the less efficient gun method. Oppenheimer directed a major reorganization of Los Alamos in July 1944 that prepared the way for the final development of an implosion bomb. Robert Bather took over G Division (G for “gadget”) to experiment with implosion and design a bomb; George Kistiakowsky led X Division (for explosives) in work on the explosive components; Hans Bethe continued to head up theoretical studies; and “Deke” Parsons (or “Deak”) now focused on overall bomb construction and delivery.

Field tests performed with uranium-235 prototypes in late 1944 eased doubts about the artillery method to be employed in the uranium bomb. It was clear that the uranium-235 from Oak Ridge would be used in a gun-type nuclear device to meet the Aug. 1, 1945, deadline Groves had given General Marshall and the Joint Chiefs of Staff. The plutonium produced at such expense and effort at Hanford would not fit into wartime planning unless a breakthrough in implosion technology occurred.

At the same time, Los Alamos shifted from research to development and production. Time was of the essence, though laboratory research had not yet charted a clear path to the final product. Army Air Force training could wait no longer, and in Sept. 1944, at Wendover Field in western Utah, Colonel Paul Warfield Tibbets Jr. began drilling the 393rd Bombardment Squadron, the heart of the 509th Composite Wing, in test drops with 5,500-pound orange dummy bombs, nicknamed “pumpkins” (nearly 50 of these practice bombs were dropped on Japan). In June 1945, Tibbets and his command moved to Tinian Island in the Marianas, where the U.S. Navy SeaBees (Construction Battalion) had built the world’s largest airport to accommodate Boeing’s new B-29 Superfortress.

Personnel shortages, particularly of physicists, and supply problems complicated Oppenheimer’s task. The procurement system, designed to protect the secrecy of the Los Alamos project, led to frustrating delays and, when combined with persistent late war shortages, proved a constant headache. The lack of contact between the remote laboratory and its supply sources exacerbated the problem, as did the relative lack of experience the academic scientists had with logistical matters.

Groves and Conant were determined not to let mundane problems compromise the bomb effort, and in fall 1944 they made several changes to prevent this possibility. Conant shipped as many scientists as could be spared from Chicago and Oak Ridge to Los Alamos, hired every civilian machinist he could lay his hands on, and arranged for Army enlisted men to supplement the work force (these GI’s were known as SEDS, for Special Engineering Detachment). Hartley Rowe, an experienced industrial engineer, provided help in easing the transition from research to production. Los Alamos also arranged for a rocket research team at the California Institute of Technology to aid in procurement, test fuses, and contribute to component development. These changes kept Los Alamos on track as weapon design reached its final stages.

Freezing Weapon Design

Weapon design for the uranium gun bomb was frozen in Feb. 1945. Confidence in the weapon was high enough that a test prior to combat use was seen as unnecessary. The design for an implosion device was approved in March 1945, with a test of the more problematic plutonium weapon scheduled for July 4, 1945. Oppenheimer shifted the laboratory into high gear and assigned Allison, Bather, and Kistiakowsky to the “Cowpuncher Committee” to “ride herd” on the implosion weapon. He placed Kenneth T. Bainbridge in charge of Project Trinity, a new division to oversee the July test firing. Parsons headed Project Alberta, known as Project A, which had the responsibility for preparing and delivering weapons for combat.

During these critical months much depended upon the ability of the chemists and metallurgists to process the uranium and plutonium into metal and craft them into the correct shape and size. Plutonium posed by far the greater obstacle. It exists in different states, depending upon temperature, and is extremely toxic. Working under intense pressure, the chemists and metallurgists managed to develop precise techniques for processing plutonium just before it arrived in quantity beginning in May 1945.

As a result of progress at Oak Ridge and metallurgical and chemical refinements on plutonium that improved implosion’s chances, the nine months between July 1944 and April 1945 saw the American bomb project progress from doubtful to probable. The Aug. 1, 1945, delivery date for the “Little Boy” uranium bomb certainly appeared more likely than it had when Groves briefed Marshall. There would be no implosion weapons in the first half of 1945 as Groves had hoped, but developments in April 1945 boded well for the scheduled summer test of the “Fat Man” plutonium bomb. And recent calculations provided by Bethe’s theoretical group gave hope that the yield for the first weapon would be in the vicinity of 5,000 tons of TNT rather than the 1,000-ton estimate provided in fall 1944. 

Here the story continues more or less word-for-word from “Manhattan - The Army and the Atomic Bomb”

Trinity - testing the atomic bomb

In many ways the climax of the Manhattan Project was Project Trinity, the crucial test, the first atomic bomb. 

Some of the scientific staff members, including Captain Parsons, strongly favored the gun rather than the implosion principle as more feasible for developing a usable fission weapon. They pointed out that the well-established mechanical techniques of the gun made this weapon type almost certain to work if properly designed and that the design and engineering of the outer configuration and mechanics of the gun were already well advanced. 

In early 1944, the laboratory intensified procurement efforts for specialized equipment for implosion testing. By then, implosion development had made giant strides, but still unknown were the relative efficiency of such a design and how long it would take to build a moderately effective implosion device.

But the sense of having achieved substantial progress in weapon design and fabrication was marred by a number of uncertainties. The feasibility of implosion had yet to be demonstrated and the rate at which uranium-235 and plutonium-239 could be produced by the Clinton and Hanford plants remained very much in question. 

Through the remaining months of 1944 and the first half of 1945, programs to perfect the uranium gun and implosion principle absorbed the major energies and resources of the reorganized laboratory. As predicted by the Los Alamos scientists, development of the gun moved ahead smoothly with few serious problems. Experiments by the laboratory's physicists proved the correctness of earlier estimates of the critical mass of the uranium-235 metal required for the gun and the gun group conducted successful firing tests, using a full sized tube and substituting uranium-238 for uranium-235.

Implosion, by way of contrast, continued to be afflicted with doubts and uncertainties. Progress toward achieving sufficient symmetry in implosion was discouragingly slow. Of the various implosion bomb designs, that proposing the use of explosive "lenses" appeared most feasible. Results of the first tests in Dec. 1944 were so unpromising that Groves and Conant concluded that uranium-235 should not be used in an implosion bomb but be conserved for the certain-to-work gun.

As the new year opened, surprising developments dispelled the lingering air of discouragement. On Feb. 27, 1945, the gun group finally had frozen design on the uranium-235 weapon, indicating a usable model would be ready by July 1945. Implosion also had made notable progress, and it was decided to manufacture the implosion model favoured by Oppenheimer. And to ensure at least one implosion bomb test with active material by 4 July 1945, Oppenheimer also decided to use the California Institute of Technology's Project Camel facilities for the construction of a second model with alternate design features. At this juncture, with data from Hanford indicating that shipments of plutonium in quantity would begin to arrive at Los Alamos in May 1945, with experiments on accurate establishment of the critical measurements on plutonium-239 in progress at the Metallurgical Laboratory, and with construction of a much larger plant for final purification of plutonium at Los Alamos well under way, the Trinity test date now appeared feasible.

Project Camel was the codename given to work performed by the California Institute of Technology (Caltech). These activities included the development of detonators, testing of bomb shapes, and the Salt Wells Pilot Plant (which I think is now part of China Lake), where explosive components of nuclear weapons were manufactured.

Project Trinity was the final step of the Los Alamos weapon program, the culmination of the laboratory's reorientation from research and experimentation to engineering, fabrication, and testing of an atomic device. Without Trinity, without the test of the bomb, the feasibility of employing the new weapon appeared to be much more questionable. "If we do not have accurate test data from Trinity", Oppenheimer and Kistiakowsky had warned, "the planning of the use of the ‘gadget’ over the enemy territory will have to be done substantially blindly". As 1945 unfolded, the Trinity mission became the central focus for the scientists at Los Alamos. With the bomb test now first priority, the tempo and intensity of Trinity preparations increased dramatically.

In the critical months of early 1945, making the “gadget” work consumed the energies of both the bomb builders and Army leaders. While the scientists worked at perfecting implosion assembly and field teams prepared the remote Trinity test site at Alamogordo, General Groves and his new deputy commander, Brig. Gen. Thomas Francis Farrell, devoted much time to overseeing Trinity preparations.

The Germans surrendered on May 7, 1945

This turned out to be the greatest crisis of conscience for the scientific community. Now, "the compelling reason for creating this weapon with such speed, our fear that Germany had the technical skill necessary to develop such a weapon, and that the German government had no moral restraints regarding its use", was gone.

The bomb was developed to stop Hitler and save the sovereign nations, and was therefore supposed to be a weapon of defense. 

Work continues in Los Alamos

As procurement crises built up in April and May 1945, Groves personally intervened in expediting requisition of lenses for the implosion bomb and globe-shaped container shells ("pumpkins") for imploding test devices. 

General Farrell represented the Army at Trinity's first major event on May 7, 1945, a rehearsal shot of 100 tons of high explosives combined with a very small amount of radioactive fission materials atop a 20-foot platform.

The 100-Ton Test, involved detonating 100 tons of TNT with radioactive material added to it, so scientists could track the airborne debris, in addition to measuring the force of the blast. When the TNT was detonated on May 7, 1945, it was the largest measured blast to that point in time.

It was a successful trial run and it gave the various Project Trinity teams practical experience in performing their assignments under difficult field conditions, demonstrated a need for improvements in the transportation and communications facilities, helped calibrate instruments, and provided a likely indication of the amount of radioactive materials needed for the final test.

Although July 4, 1945, had been set as the target date for the test, few scientists at Los Alamos were convinced it could be met. Precise scheduling depended upon bringing a tremendous number of factors into proper juxtaposition, including weather, procurement of key components and equipment, production and shipment of active material, preparation of many experiments, and arrangement of security and safety measures. In mid-June 1945, Oppenheimer announced to the laboratory's group leaders that July 13, 1945, was the earliest possible date, with up to ten days later not unreasonable. He based his estimate upon information provided by the laboratory's “Cowpuncher Committee”, which had primary responsibility for coordination and scheduling of Trinity.

Following another review of developments on June 30, 1945, this committee advanced the test date to July 16, 1945, to permit inclusion of certain additional vital experiments. Two days later, Oppenheimer indicated to Groves that the laboratory leaders finally had agreed on the July 17, 1945. Groves, however, objected to the later date, pointing out that the situation in Washington required an earlier date.

With the end of the war in Europe, Secretary Stimson was scheduled to depart in early July 1945, for the Potsdam Conference, with sessions starting on the July 16, 1945. The Manhattan commander undoubtedly had conferred with Conant, Tolman, and Stimson's assistants, George Leslie Harrison and Harvey Hollister Bundy, all of whom favoured carrying out the test on the July 14, 1945. Final preparations advanced well, and Oppenheimer called Groves fixing the date for the test on the July 16, 1945.

All systems go!

On July 12, 1945, two scientists from Los Alamos arrived in an Army sedan with the plutonium-239 core for the implosion device. The next day a convoy came from the Hill with the non-nuclear components, including the high explosives. With all components in place except the detonating system, workers lifted the device to a metal shed on a platform at the top of the tower. The detonator group then completed the firing circuit and other technicians added the instrumentation for the experiments. By five in the afternoon of the July 14, 1945, the device was ready for the test.

The next day, a Sunday, Trinity crews carried out last-minute inspections and observers checked into the Base Camp, about 10 miles south of the test tower. OSRD Director Vannevar Bush and Conant arrived from Pasadena with General Groves. Army sedans brought Charles Thomas from Santa Fe and Ernest Lawrence, Sir James Chadwick, and New York Times science reporter William L. Laurence, as well as others, from Albuquerque. Compton had decided not to come. Tolman and General Farrell were already on hand. The large contingent from Los Alamos, aboard three buses, did not reach Trinity until shortly before 03:00 of July 16, 1945, barely in time for the originally scheduled zero hour, 04:00. They stepped out into blustery and rainy weather with occasional flashes of lightning, not the clear skies and moderate winds the Trinity meteorologists had predicted.

Oppenheimer and Groves had reviewed the weather situation at midnight and then had gone forward from the Base Camp some 7,000 yards to the control dugout (10,000 yards from the test tower) to wait with Farrell, physicist Kenneth Bainbridge, who was the leader of the bomb test team, and chief meteorologist Jack M. Hubbard, who with Oppenheimer had responsibility for making the final decision on whether to carry out the test as scheduled. As 04:00 approached and the rain continued, Groves and Oppenheimer weighed the risks of going ahead, the likelihood of heavier radioactive fallout at some points, electrical failures from dampened circuits, and poor visibility for the observation airplanes. They decided to delay the shot an hour and a half. The rain stopped at four and shortly before five, with wind still blowing in the right direction, they gave the go-ahead signal for the test.

As the final countdown began, Groves left Oppenheimer and Farrell in the control dugout and returned to the Base Camp, a better point of observation and in compliance with the rule that he and Farrell must not be together in situations where there was an element of danger. At approximately the same time, the five Trinity scientists who had been guarding the test device drove away in their jeeps as bright lights illuminated the tower to foil any would-be saboteurs. 

Precisely at 05:30, an automatic firing mechanism actuated the implosion device. Data from hundreds of instruments recorded what occurred in that desolate stretch of the Jornada del Muerto valley: the dawn of the atomic age. It began with a brilliant yellow light that suffused the remotest recesses of the Trinity site and was seen as far away as Albuquerque and Los Alamos to the north, Silver City (New Mexico) to the west, and El Paso (Texas) to the south. With the light came a sensation of heat that persisted even as a huge ball of fire, like a rising sun, took shape, then transformed quickly into a moving orange and red column. Out of this broad spectrum of colours rose a narrower column that rapidly spilled over to form a giant white mushroom cloud surrounded by a blue glow. Only as the glow began to fade did observers at the base camp feel the pressure of the shock wave, but its rumble reverberated for more than five minutes in the surrounding hills.

The effects of this explosion on eyewitnesses were as varied as the observers themselves. What General Farrell, for example, saw and heard from the control dugout was "unprecedented, magnificent, beautiful, stupendous and terrifying. . . . The whole country was lighted by a searing light with the intensity many times that of the midday sun. It was golden, purple, violet, gray and blue. It lighted every peak, crevasse and ridge of the nearby mountain range with a beauty . . . the great poets dream about. . . . Thirty seconds after, the explosion came . . . followed almost immediately by the strong, sustained, awesome roar which warned of doomsday. . . ."  

The “Gadget” partially uncovered, and complete, sitting on the top of the tower. 

Groves permitted himself only a fleeting moment of relaxation. Less than half an hour after the test shot he called his secretary in Washington, D.C., to inform George Harrison so that he could pass on word of the test to Stimson in Potsdam. Groves reported that the strength of the explosion was at least "satisfactory plus" and perhaps far greater than estimated.  Stimson, Truman, Churchill, and other Allied leaders at Potsdam were quick to realize that this preliminary evidence of the enormous power of the Trinity explosion, followed soon by more detailed substantiating data from General Groves, had introduced a new factor that would profoundly affect not only their own deliberations on how to end the war with Japan but also the whole course of international relations in the postwar world.

What a successful trial meant

The explosion of an implosion device on July 16, 1945, at Trinity provided final confirmation to America's wartime leaders that employment of an atomic weapon in the war with Japan was indeed a strategic reality. Until 1945, the Army's super secret atomic weapon program had not been a factor in strategic planning for carrying on the war, either in Europe or in the

Pacific. The successful Allied operations against Germany in the summer of 1944 portended that country's imminent collapse and obviated the need for an atomic weapon to end the conflict in Europe. Because of these developments, Manhattan Project leaders thus considered using the bomb in the war in the Pacific and accelerated preliminary planning with the U.S. Army Air Forces (an independent U.S. Air Force was only created in 1947)  for a possible atomic bombing mission against Japan.

Preparations for the tactical employment of an atomic weapon against Japan had already began in late March 1944, when General Groves first met with General Henry Harley Arnold, the AAF commanding general. The Manhattan commander briefed Arnold on the current status of bomb development, and estimating the probable time when bombs would be ready for use in combat. The next question was what type of airplane would be required to transport atomic bombs. On the basis of investigations carried out at Los Alamos and Muroc Army Air Field, had concluded that a modified B-29 probably had the requisite weight-carrying capacity and range. Should the B-29, which had gone into production in Sept. 1943, prove not feasible, Groves suggested the British Lancaster would have to be considered. This displeased Arnold, who stated emphatically that an American-made airplane should carry the bombs, and he promised to make a special effort to have a B-29 available for that purpose.

So the AAF would organize and train the requisite tactical bomb unit, which, for reasons of security, was to be as self-sustaining as possible and exercise full control over delivery of bombs on the targets selected. Manhattan would receive from the AAF whatever assistance it needed in ballistic testing of bombs and air transportation of materials and equipment. Pending completion of the fission bombs, Groves assured Wilson that, for testing purposes, Manhattan would supply the AAF with several hundred high-explosive bombs having ballistic characteristics similar to the implosion-type model.

The AAF committed itself to supply the personnel and equipment for a heavy bomb squadron, with attached special units as required, and to make available an air base in the southwestern United States for its training. In addition, it agreed to modify and complete delivery of fourteen B-29's to the squadron by Jan. 1 1945; to continue flight testing of implosion-type bombs, with related training under direction of Manhattan and AAF specialists; and to assist Manhattan personnel in testing equipment and assembling ballistic data. 

To command the bomb combat unit, subsequently designated the 509th Composite Group and formally activated on Dec. 17 1944, General Arnold selected Col. Paul W. Tibbets, Jr. Tibbets had an outstanding record in flying heavy bombers in Europe and North Africa and had gained a special knowledge of the B-29 as a test pilot. Because of the great importance and secrecy of the 509th's mission, Arnold gave the 509th commander virtual carte blanche to select the best-qualified personnel available.

In Sept. 1944, Colonel Tibbets began to assemble the elements of the 509th at Wendover Field, an isolated air base in western Utah with adequate security and facilities and well located for air travel to Los Alamos and the Salton Sea Naval Air Station. Tibbets formed the various elements of the 509th with the objective of making it as self-sufficient as possible. Thus, he included in the group not only a normal B-29 unit, the 393d Bombardment Squadron (VH), but also a number of supporting elements, including the 390th Air Service Group (consisting of the 603d Air Engineering and 1027th Materiel Squadrons), the 320th Troop Carrier Squadron, and the 1395th Military Police Company (Aviation). Subsequently, for special technical requirements, the 509th acquired the 1st Ordnance Squadron, Special (Aviation), and the 1st Technical Detachment, War Department Miscellaneous Group, a catchall unit comprised of both civilian and military scientists and technicians, many from the Manhattan Project but including Army, Navy, and AAF personnel.

At the beginning of Sept. 1944, with the external shape and aircraft requirements of the three basic bomb models, one of the uranium-235 gun type (now designated “Little Boy” instead of “Thin Man”) and two of the plutonium-239 implosion type (“Fat Man”), now frozen, the AAF started training the bomb drop squadron and, with assistance from Los Alamos technicians, completed necessary modifications on the B-29. While awaiting delivery of the first planes the squadron underwent training that emphasized ground and air techniques for handling atomic bombs.

In Oct. 1944, only days past the scheduled delivery date, the 393d received the first modified B-29's out of a production lot of fifteen (one more than originally requested). Without delay, a continuing series of essential test drops commenced at Wendover. Over the next few months, these tests furnished critical information on ballistics, electrical fusing, flight performance of electrical detonators, operation of aircraft release mechanisms, vibration, and temperatures, as well as provided bomb assembly experience. But, perhaps more importantly, they revealed certain weaknesses in the original modifications and defective performance in the flying capabilities of the big bombers.

Because B-29's were in very short supply, the AAF's lower echelons displayed some reluctance to satisfy the Manhattan request for replacement of the inadequate planes. General Arnold said that the 509th Composite Group would get as many new planes as it required. Finally, in the spring of 1945, the second lot of fifteen greatly improved versions of the B-29 reached the air base, and training and ballistic tests proceeded at a more intensive pace.

Project leaders turned their attention to establishing a base of operations for the 509th in the Pacific Theater. The AAF recommended that leaders of the Twentieth Air Force in the Marianas, at the time the only feasible location for the 509th base, be informed of the atomic bomb mission. It was also necessary to inform the Navy commanders in the Pacific of the atomic bomb mission, as Navy support in the immediate area of operations would be indispensable. Furthermore, Admiral Chester William Nimitz, Commander in Chief, Pacific Ocean Areas (CINCPOA), had learned of the imminent arrival of the 509th in his theater and was asking questions concerning its mission. 

The airfield and port facilities under construction on Tinian were more than adequate for the atomic bomb mission and would be ready for use by the time the 509th arrived in June 1945. Furthermore, although the Army had jurisdiction over Tinian, the Navy's 6th Naval Construction Brigade was available there to build the special installations that would be needed by the mission. By end of the month, U.S. Navy Seabees were at work on the base facilities.

By mid-July, 1945, all elements of the group had reached Tinian, including the 1st Technical Detachment comprised chiefly of civilian specialists from Los Alamos, some of whom had been brought temporarily into military service. Commanded by Parsons, the detachment furnished and tested weapon components for the 509th, supervised assembly of bombs, and checked out completed units, carefully inspecting them in bomb bays before planes took off. 

The 509th's combat crews continued intensive flight training. This involved practicing navigation missions to Iwo Jima and making bomb runs to nearby islands still in enemy hands, using high-explosive projectiles with the “pumpkin” shape. At the end of training, which lasted three weeks, the crews in late July 1945, began a series of combat strikes over Japan to gain familiarity with target areas and mission tactics and also to accustom the Japanese to the appearance of small formations of B-29's flying at a great altitude. Using the pumpkin-shaped bombs, the 509th achieved excellent results against enemy towns, most of which had been hit by previous B-29 strikes. These towns, Koriyama, Nagaoka, Toyama, Kobe, Yokkaichi, Ube, Wakayama, Maizuru, Fukushima, and Niihama, were in the general vicinity of those communities selected earlier as targets for atomic bombing.

The bombing targets

In the late spring and early summer of 1945, Manhattan and AAF representatives met in Washington and Los Alamos for the purpose of choosing targets for the 509th's atomic bombing mission.  Groves, after conferring with General Arnold, and with General Lauris Norstad selected a target committee. The committee included two members of Groves's staff (General Farrell, who served as de facto chairman when Groves was not present, and Major John A. Derry), an AAF officer (Col. William P. Fisher), and five technical experts (John von Neumann, Robert R. Wilson, and William G. Penney, a member of the British team at Los Alamos, all from the Manhattan Project, and Joyce Clennam Stearns and David Mathias Dennison from the AAF.

Three meetings were held. The committee carefully considered various criteria: the maximum range for the loaded B-29 aircraft; the need for visual bombing; likely weather conditions; and expected damage. The last criterion weighed heavily on the committee, for it pointed to the necessity to select targets where the bomb would produce the maximum damage and hence have the profoundest impact upon enemy morale. Project scientists had indicated that the bomb would most likely achieve the desired results if it were dropped on densely built-up areas of significant value to the Japanese war effort. They also had emphasized that the targets should not have been bombed previously, so the effects might be assessed more accurately. The choices were Kokura Arsenal, one of Japan's largest munitions plants, covering an area of 8 million square feet; Hiroshima, a major military embarkation port and convoy assembly point with a local army headquarters, railway yards, storage depots, and some heavy industrial plants; Niigata, an important seaport with significant industrial and commercial facilities, including an aluminum reduction plant, a large ironworks, an oil refinery, and a tanker terminal; and Kyoto, with a concentrated 3-square-mile industrial area and a population of about one million people. Stimson expressed strong objection to Kyoto, noting that the city had been the ancient capital of Japan and was a place of great religious and cultural significance to the Japanese.

When the atomic bomb directive was issued to the United States Army Strategic Air Forces (USASTAF) on July 25, 1945 Nagasaki had replaced Kyoto on the target list.

The decision to use the bomb

Meanwhile, the question of military employment of the bomb against Japan came up for consideration by the Interim Committee, a temporary body appointed by Stimson in May 1945 at the urging of project leaders and with the approval of the President.

Membership was comprised of the Secretary of War (chairman); George Harrison, as alternate chairman; former War Mobilization Director James F. Byrnes, representing the President; Vannevar Bush; James B. Conant; MIT President Karl T. Compton; Assistant Secretary of State for Economic Affairs William Lockhart Clayton; and Under Secretary of the Navy Ralph Austin Bard. The Interim Committee established a scientific panel, comprised of Oppenheimer, Fermi, Arthur Compton, and Lawrence. While recognizing that use of the bomb was essentially a military matter, the panel members nevertheless offered their opinions concerning the way it should be employed and the likely effects it would have on the targets selected. 

Taking a moment to reflect on the discussion of targets and effects, Secretary Stimson proffered the conclusion that the atomic bomb should be used against Japan with no advance warning and, while not restricting the target to a civilian area, should be employed in such a way as "to make a profound psychological impression on as many of the inhabitants as possible". Conant suggested that the "most desirable target would be a vital war plant employing a large number of workers and closely surrounded by workers' houses". 

Among the various reports Compton received in the following two weeks was one prepared by a group of scientists under the leadership of James Franck, an outstanding German-refugee physicist who had come to the Metallurgical Laboratory from the staff of the University of Chicago. Centering on the political and social ramifications of an atomic bombing, the Franck report favored eventual international control of atomic energy as the only safe solution. Using the bomb against Japan without adequate warning, the report cautioned, would arouse great animosity against the United States and isolate her morally among the nations of the world, making establishment of international controls much more difficult. As an alternative, the report advocated a demonstration of the bomb in an uninhabited area, pointing out that this action would not prevent later military use of the bomb against Japan, if this were necessary.

On July 21, 1945, Stimson received not only Groves's detailed report on the successful test at Trinity, but also cables from Harrison indicating that atomic bombs would be ready sooner than expected. He promptly passed the word to American and British leaders at the Potsdam Conference, including President Truman, Prime Minister Churchill, Secretary of State Byrnes (as of 3 July), General Marshall, and Lord Cherwell, all of whom were elated by the news. On July 24, 1945, Stimson showed the President the tentative plan of operations, which called for the first atomic bombing mission any time after Aug 1, 1945, subject to completion of preparations and suitable weather. Truman accepted the plan without reservation, for, Stimson recalled, "that was just what he wanted. . . ."

On July 25, 1945, it was decided to proceed with the atomic bombing of Japan, and the procedure was that the 509th Composite Group, 20th Air Force was to deliver its first special bomb as soon as weather permited visual bombing after about Aug. 3, 1945 on one of the targets: Hiroshima, Kokura, Niigata or Nagasaki. And additional bombs were to be delivered on the above targets as soon as made ready by the project staff. 

On July 26, 1945 Truman and other Allied leaders issued the Potsdam Declaration outlining terms of surrender for Japan. It was presented as an ultimatum and stated that without a surrender, the Allies would attack Japan, resulting in "the inevitable and complete destruction of the Japanese armed forces and just as inevitably the utter devastation of the Japanese homeland". The atomic bomb was not mentioned in the communication. On July 28, 1945, Japanese papers reported that the declaration had been rejected by the Japanese government. The statement was taken by both Japanese and foreign papers as a clear rejection of the declaration. On Aug. 6 and 9, 1945 the atomic bombs were released on Hiroshima and Nagasaki causing catastrophic wide spread devastation of Japan. On Aug. 15, 1945, Japan surrendered to the Allied leaders.

Survey of the bombing effects

The swift surrender of Japan opened the way for American scientific teams to survey, on the ground, the specific effects of the atomic bombing of Hiroshima and Nagasaki. Not only were scientists, medical personnel, and professional military men greatly interested in learning the results of the first employment of atomic weapons in warfare, but also the commanders of the occupation troops that were scheduled shortly to move into the two bombed cities desired a check on the possible hazards with which they might have to cope. 

Negotiations with the Japanese to arrange for an early entry into Hiroshima and Nagasaki culminated in the formation of a special party, comprised mostly of medical personnel from the International Red Cross, the Army Medical Corps, MacArthur's staff, and the Manhattan Project. The special party, accompanied by two representatives of the Japanese government, flew into Hiroshima on Sept. 8, 1945. Using Geiger counters and other instruments, members of the party checked through the destroyed area of the city, determining that no significant amounts of radioactivity persisted. 

Within a radius of 1 mile of the epicenter of the explosion, destruction in both cities was virtually complete, except for the frames of a few reinforced concrete buildings. 

A primary objective of the Manhattan survey teams was to ascertain the particular kinds of injuries suffered, with special attention to the effects of radioactivity. By far the largest number of casualties resulted from burns traceable to the heat of the explosion and the fires generated by it and secondary causes. Other major sources of injury were falling debris, pressure of the blast, and radiation.

Most radiation injuries occurred from exposure of the victims to gamma rays at the time of the explosion. There was little evidence of casualties from alpha and beta rays and from residual radioactivity in the bombed-out areas.

It was also concluded that, while the bombs had some impact on the leaders of the Japanese government, their knowledge of the awesome character of the new weapon seemed not to have played a significant part in convincing them of the need to surrender.

Yet atomic bombs are still considered to have played a crucial role in ending the war. But as Gerald James Holton (professor of the history of science at Harvard) once commented, "What really won the war was radar. It was radar and the proximity fuse and above all, because it is so forgotten, synthetic rubber, without which the war could not be pursued. As a result of these successes in essentially technologies the scientists and engineers had a completely different status". One also could add sonar, aerodynamics, the internal combustion engine and communications technology to that list.


What is certain is that the Manhattan Project put “Big Science” on the map, and the atomic bomb put the physicist on top of the pile.  © Bernard Smith 2017-18