The Manhattan Project was a research and development program undertaken during World War II to produce the first nuclear weapons. It was led by the United States in collaboration with the United Kingdom and Canada. The Manhattan Project employed nearly 130,000 people at its peak and cost nearly US$2 billion (equivalent to about $28 billion in 2025).
From 1942 to 1946, the project was directed by Major General Leslie Groves of the U.S. Army Corps of Engineers. Nuclear physicist J. Robert Oppenheimer was the director of the Los Alamos Laboratory that designed the bombs. The Army program was designated the Manhattan District, as its first headquarters were in Manhattan; the name superseded the initial codename, Development of Substitute Materials, for the entire project. The project absorbed its earlier British counterpart, Tube Alloys.
Over 80 percent of project cost was for building and operating the fissile material production plants. The project proposed both highly enriched uranium and plutonium as fuel for nuclear weapons. Enriched uranium was produced at the Clinton Engineer Works in Tennessee. Plutonium was produced in the world's first industrial-scale nuclear reactors at the Hanford Engineer Works in Washington. These sites were supported by dozens of other facilities across Canada, the US and the UK.
The weapons' designs were produced at the Los Alamos Laboratory (Project Y) in New Mexico, and resulted in two weapon designs used during the war: Little Boy (enriched uranium gun-type) and Fat Man (plutonium implosion). The Fat Man design was initially a low priority fallback option, as it was complex and required explosive lenses, but in 1944 it was confirmed that plutonium from Hanford was not suitable for a gun-type bomb because of the high proportion of plutonium-240. The first nuclear device ever detonated was an implosion-type bomb during the Trinity test, conducted at White Sands Proving Ground in New Mexico on 16 July 1945. The project was responsible for developing the specific means of delivering the weapons onto military targets, and for the use of the Little Boy and Fat Man bombs in the atomic bombings of Hiroshima and Nagasaki in August 1945.
The project was also charged with gathering intelligence on the German nuclear weapon project. Through Operation Alsos, Manhattan Project personnel served in Europe, sometimes behind enemy lines, where they gathered nuclear materials and documents and rounded up German scientists. Despite the Manhattan Project's own emphasis on security, Soviet atomic spies penetrated the program.
In the immediate postwar years, the Manhattan Project conducted weapons testing at Bikini Atoll as part of Operation Crossroads, developed new weapons, promoted the development of the network of national laboratories, supported medical research into radiology, and laid the foundations for the nuclear navy. It maintained control over American atomic weapons research and production until the formation of the United States Atomic Energy Commission (AEC) in January 1947.
Origins
Uranium Committee
The discovery of nuclear fission by Otto Hahn and Fritz Strassmann in 1938, and its theoretical explanation by Lise Meitner and Otto Frisch, made an atomic bomb using uranium theoretically possible. There were fears that a German atomic bomb project would develop one first. In August 1939, Leo Szilard and Eugene Wigner drafted the Einstein–Szilard letter, which warned of the potential development of "extremely powerful bombs of a new type". It urged the United States to acquire stockpiles of uranium ore and accelerate the research of Enrico Fermi and others into nuclear chain reactions. They had it signed by Albert Einstein and delivered to President Franklin D. Roosevelt.
Roosevelt called on Lyman Briggs to head an Advisory Committee on Uranium; Briggs met with Szilard, Wigner and Edward Teller in October 1939. The committee reported back to Roosevelt in November that uranium "would provide a possible source of bombs with a destructiveness vastly greater than anything now known." In February 1940, the U.S. Navy awarded Columbia University $6,000, most of which Fermi and Szilard spent on graphite. A team of Columbia professors including Fermi, Szilard, Eugene T. Booth and John Dunning created the first nuclear fission reaction in the Americas, verifying the work of Hahn and Strassmann. The same team subsequently built a series of prototype nuclear reactors (then called "atomic piles," a term coined by Fermi) in Pupin Hall at Columbia but were not yet able to achieve a chain reaction.
The work of Briggs' committee was seen as insufficiently urgent by some of the scientists who were familiar with it. When the National Defense Research Committee (NDRC) was formed on 27 June 1940 to coordinate scientific research, the Advisory Committee on Uranium was absorbed into the organization. On 28 June 1941, Roosevelt signed Executive Order 8807, which created the Office of Scientific Research and Development (OSRD), under director Vannevar Bush. The office was empowered to engage in research and large engineering projects, and was immediately converted into the OSRD Section on Uranium. In July 1941, Briggs proposed spending $167,000 on researching uranium, particularly the uranium-235 isotope, and plutonium, which had been isolated for the first time at the University of California in February 1941. The work, however, was still being conducted at a relatively small scale and scope, as Bush and other administrators were still skeptical that the work could achieve quick success.
British intervention
In Britain, Otto Robert Frisch and Rudolf Peierls at the University of Birmingham had made a breakthrough investigating the critical mass of uranium-235 in June 1939. Their calculations indicated that it was within an order of magnitude of 10 kilograms (22 lb), small enough to be carried by contemporary bombers. Their March 1940 Frisch–Peierls memorandum initiated the British atomic bomb project and its MAUD Committee, which unanimously recommended pursuing the development of an atomic bomb. In July 1940, Britain had offered to give the United States access to its research, and the Tizard Mission's John Cockcroft briefed American scientists on British developments. He discovered that the American project was smaller than the British, and not as advanced.
As part of the scientific exchange, the MAUD Committee's findings were conveyed to the United States. One of its members, the Australian physicist Mark Oliphant, flew to the US in late August 1941 and discovered that data provided by the MAUD Committee had not reached key American physicists. Oliphant set out to find out why the committee's findings were apparently being ignored; the report had been received by Briggs, who had locked it in a safe. Oliphant met with the Uranium Committee and visited Berkeley, California, where he spoke persuasively to Ernest O. Lawrence. Lawrence was sufficiently impressed to commence his own research. He in turn spoke to James B. Conant, Arthur H. Compton and George B. Pegram. Oliphant's mission was a success; key American physicists were now aware of the wartime feasibility of an atomic bomb.
On 9 October 1941, during a meeting with Vannevar Bush about the British conclusions, President Roosevelt approved accelerating the atomic program. He created a Top Policy Group consisting of himself—although he never attended a meeting—Vice President Henry A. Wallace, Bush, Conant, Secretary of War Henry L. Stimson, and the Chief of Staff of the Army, General George C. Marshall. Roosevelt chose the Army to run the project rather than the Navy, because the Army had more experience with managing large-scale construction. He agreed to coordinate the effort with the British and on 11 October sent a message to Prime Minister Winston Churchill, suggesting that they correspond on atomic matters. Bush had, by this point, already begun expanding and altering the leadership of the OSRD Section on Uranium, retaining Briggs as chairman but adding and elevating other personnel. The section itself was renamed to Section S-1.
S-1 Committee
The S-1 Committee meeting on 18 December 1941 was "pervaded by an atmosphere of enthusiasm and urgency" in the wake of the attack on Pearl Harbor and the United States declaration of war on Japan and on Germany. Work was proceeding on three techniques for isotope separation: Lawrence and his team at the University of California investigated electromagnetic separation, Eger Murphree and Jesse Wakefield Beams's team looked into gaseous diffusion at Columbia University, and Philip Abelson directed research into thermal diffusion at the Carnegie Institution of Washington and later the Naval Research Laboratory. Murphree was also supervisor of a separation project, directed by Beams, investigating the use of gas centrifuges. Meanwhile, there were two lines of investigation into nuclear reactor technology, with different neutron moderators: Harold Urey researched heavy water reactors at Columbia, while Arthur Compton organized the Metallurgical Laboratory at the University of Chicago in early 1942 to study plutonium and reactors moderated by graphite.
The S-1 Committee recommended pursuing all five technologies simultaneously. This was approved by Bush, Conant, and Brigadier General Wilhelm D. Styer, who had been designated the Army's representative on nuclear matters. Bush and Conant then took the recommendation to the Top Policy Group with a budget proposal for $54 million for construction by the United States Army Corps of Engineers, $31 million for research and development by OSRD and $5 million for contingencies in fiscal year 1943. They cited the fear of a German atomic bomb as a justification. The proposal recommended that new sites for research and production be selected, that production priorities be established, and that the entire project be put under strict security regulations. On 17 June 1942, President Roosevelt approved the recommendations.
Organization
Manhattan District
The Chief of Engineers, Major General Eugene Reybold, selected Colonel James C. Marshall to head the Army's part of the project in June 1942. Marshall created a liaison office in Washington, D.C., but established his temporary headquarters at 270 Broadway in New York, where he could draw on administrative support from the Corps of Engineers' North Atlantic Division. It was close to the Manhattan office of Stone & Webster, the principal project contractor, and to Columbia University. He had permission to draw on his former command, the Syracuse District, for staff, and he started with Lieutenant Colonel Kenneth Nichols, who became his deputy.
Because most of his task involved construction, Marshall worked in cooperation with the head of the Corps of Engineers Construction Division, Major General Thomas M. Robins, and his deputy, Colonel Leslie Groves. Reybold, Somervell, and Styer decided to call the project "Development of Substitute Materials", but Groves felt that this would draw attention. Since engineer districts normally carried the name of the city where they were located, Marshall and Groves agreed to name the Army's component the Manhattan District; Reybold officially created this district on 13 August. Informally, it was known as the Manhattan Engineer District, or MED. Unlike other districts, it had no geographic boundaries, and Marshall had the authority of a division engineer. Development of Substitute Materials remained as the official codename of the project as a whole but was supplanted over time by "Manhattan".
Marshall later conceded that, "I had never heard of atomic fission but I did know that you could not build much of a plant, much less four of them for $90 million." A single TNT plant that Nichols had recently built in Pennsylvania had cost $128 million. Nor were they impressed with estimates to the nearest order of magnitude, which Groves compared with telling a caterer to prepare for between ten and a thousand guests. A survey team from Stone & Webster had already scouted a site for the production plants. The War Production Board recommended sites around Knoxville, Tennessee, an isolated area where the Tennessee Valley Authority could supply ample electric power and the rivers could provide cooling water for the reactors. After examining several sites, the survey team selected one near Elza, Tennessee. Conant advised that it be acquired at once, Styer agreed, but Marshall temporized, awaiting the results of Conant's reactor experiments. Of the prospective processes, only Lawrence's electromagnetic separation appeared sufficiently advanced for construction to commence.
Marshall and Nichols began assembling the necessary resources. The first step was to obtain a high priority rating for the project. The top ratings were AA-1 through AA-4 in descending order, although there was a special AAA rating reserved for emergencies. Ratings AA-1 and AA-2 were for essential weapons and equipment, so Colonel Lucius D. Clay, the deputy chief of staff at Services and Supply for requirements and resources, felt that the highest rating he could assign was AA-3, although he was willing to provide a AAA rating on request for critical materials if the need arose. Nichols and Marshall were disappointed; AA-3 was the same priority as Nichols' TNT plant in Pennsylvania.
Berkeley summer conference
In the spring of 1942, Arthur Compton asked theoretical physicist J. Robert Oppenheimer of the University of California to take over research into fast neutron calculations—key to calculations of critical mass and weapon detonation—from Gregory Breit, who had quit on 18 May 1942 because of concerns over lax operational security. John H. Manley, a physicist at the Metallurgical Laboratory, was assigned to assist Oppenheimer by coordinating experimental physics groups scattered across the country. Oppenheimer and Robert Serber of the University of Illinois examined the problems of neutron diffusion—how neutrons moved in a nuclear chain reaction—and hydrodynamics—how the explosion produced by a chain reaction might behave.
To review this work and the general theory of fission reactions, Oppenheimer and Fermi convened meetings at the University of Chicago in June and at the University of California in July 1942 with theoretical physicists Hans Bethe, John Van Vleck, Edward Teller, Emil Konopinski, Robert Serber, Stan Frankel, and Eldred C. (Carlyle) Nelson, and experimental physicists Emilio Segrè, Felix Bloch, Franco Rasetti, Manley, and Edwin McMillan. They tentatively confirmed that a fission bomb was theoretically possible.