The Manhattan Project was a research and development undertaking during World War II that produced the first nuclear weapons. It was led by the United States with the support of the United Kingdom and Canada. Research and production took place at more than 30 sites across the United States, the United Kingdom, and Canada. The Einstein-Szilard letter prompted the president to initiate the project under the direction of the S-1 Executive Committee. It was subsequently merged with the British and Canadian projects by the 1943 Quebec Agreement.
Two types of atomic bombs were developed concurrently. The Thin Man gun-type design proved impractical to use with plutonium, and a simpler gun-type called Little Boy was developed that used uranium-235, an isotope that makes up only 0.7 percent of natural uranium. Chemically identical to the more abundant uranium-238, and with almost the same mass, it proved difficult to separate the two. Three methods were employed: electromagnetic in the calutrons, gaseous by the K-25 Project, and thermal by the S-50 Project. Most of this work was performed at the Clinton Engineer Works at Oak Ridge, Tennessee. Means for refining uranium were developed by the Ames Project.
In parallel with the work on uranium was an effort to produce plutonium. After its feasibility was demonstrated by Chicago Pile-1, the world's first artificial nuclear reactor, the Metallurgical Laboratory in Chicago designed the X-10 Graphite Reactor at Oak Ridge and the production reactors at the Hanford Site, in which uranium was irradiated and transmuted into plutonium. This was then chemically separated from the uranium. The Fat Man implosion-type weapon using plutonium was developed in a concerted design and development effort by Project Y at Los Alamos, New Mexico. Heavy water for use in reactors designed by the Metallurgical Laboratory and the Canadian Montreal Laboratory was produced by the P-9 Project. Polonium was produced by the Dayton Project. The Manhattan Project was also charged with gathering intelligence through Operation Alsos, and defense against radioactive weapons under Operation Peppermint.
The first nuclear device ever detonated was a Fat Man bomb at the Trinity test on 16 July 1945. Little Boy and Fat Man bombs were used a month later in the atomic bombings of Hiroshima and Nagasaki. The formerly secret project was made public by the Smyth Report. In the immediate postwar years, the Manhattan Project assisted weapons testing in Operation Crossroads. It maintained control over American atomic weapons research and production until January 1947, when the Atomic Energy Act of 1946 took effect.
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