nuclear fission German atomic bomb project Einstein–Szilard letter MAUD Committee isotopically uranium-235 neutron moderator heavy water U.S. Army Corps of Engineers plutonium-239 Manhattan Project Leslie R. Groves, Jr. Major General General plutonium
The discovery of nuclear fission in 1938 made the development of an atomic bomb a theoretical possibility. Fears that a German atomic bomb project would develop atomic weapons first, especially among scientists who were refugees from Nazi Germany and other fascist countries, were expressed in the Einstein–Szilard letter to Roosevelt in 1939. This prompted preliminary research in the United States in late 1939. Progress was slow until the arrival of the British MAUD Committee report in late 1941, which indicated that only 5 to 10 kilograms of isotopically-pure uranium-235 were needed for a bomb instead of tons of natural uranium and a neutron moderator like heavy water. Consequently, the work was accelerated, first as a pilot program, and finally in the agreement by Roosevelt to turn the work over the U.S. Army Corps of Engineers to construct the production facilities necessary to produce uranium-235 and plutonium-239. This work was consolidated within the newly-created Manhattan Engineer District, which became better known as the Manhattan Project, eventually under the direction of Major General Leslie R. Groves, Jr..
