The product nuclei emit several neutrons (in 1943 nobody knew exactly how many, on the aver¬age) for every U-235 nucleus split. Other U-235 nuclei may absorb these neutrons and undergo fission in turn, producing still more neutrons. Such a chain reaction will proceed very rapidly, as long as one neutron (at least) from each fission causes another fission.
Theoretically, the energy release from one kilo¬gram (about 2.2 pounds) of U-235 would equal the energy release from the detonation of 17,000 tons of TNT.
Given these facts, the problem was to devise a bomb deriving its explosive energy from the fission of uranium-235 (or of plutonium-239, the only other fissionable material under consideration in 1943).
Neither U-235 nor Pu-239 was available in suffi¬cient quantity to make a bomb. It was thought that these materials would become available within two years. The task of the Los Alamos staff was to design the bomb, and to devise methods of manu-facturing it, in advance of the scheduled deliveries of the needed material.
