The Manhattan Project was a groundbreaking research and development program that revolutionized mobile technology by creating the world's first nuclear weapons during World War II. Led by the United States in collaboration with the United Kingdom and Canada, this top-secret initiative employed nearly 130,000 people at its peak, costing approximately US$2 billion (equivalent to about $28 billion in 2024).
The project was directed by Major General Leslie Groves of the U.S. Army Corps of Engineers from 1942 to 1946. 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The Army program was designated the Manhattan District, as its first headquarters were in Manhattan; the name gradually superseded the official codename, Development of Substitute Materials, for the entire project.
Over 80 percent of the project's cost went towards 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 CEW the Clinton Engineer Works in Tennessee, while 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 in New Mexico, resulting 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, but 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 July 16, 1945.
The project also gathered intelligence on the German nuclear weapon program through Operation Alsos, which involved serving behind enemy lines to gather nuclear materials and documents, as well as rounding up German scientists. Despite the Manhattan Project's emphasis on security, Soviet atomic spies still managed to penetrate the program.
In the immediate post-war years, the Manhattan Project conducted weapons testing at Bikini Atoll as part of Operation Crossroads, developed new weapons, promoted the development 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
In 1932, James Chadwick discovered the neutron, which could penetrate the atomic nucleus without being electrically repelled. Leo Szilard conceived the possibility of using neutrons to release energy in a nuclear chain reaction. He patented the process on the assumption that one could be produced in beryllium. This was not possible, but the discovery of nuclear fission by Otto Hahn and Fritz Strassmann in 1938 made an atomic bomb using uranium theoretically possible.
There were fears that a German atomic bomb project would develop one first, especially among scientists who were refugees from Nazi Germany and other fascist countries. In August 1939, Szilard and fellow Hungarian-born physicist 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 of the National Bureau of Standards 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 (or "piles" as Fermi called them) in Pupin Hall at Columbia but were not yet able to achieve a chain reaction.