One of the early discoveries relative to nuclear fission was that neutrons are one product of the fission reaction. That is, although a neutron is needed to initiate a fission reaction, at least one neutron is also produced as a result of the reaction. Some scientists realized that this fact meant that fission reactions could easily become chain reactions . A chain reaction is one in which the substance needed to start the reaction is also produced as a result of the reaction.

Enrico Fermi (1901-1954), Leo Szilard, and other physicists working in the Manhattan Project realized the practical significance of a nuclear fission chain reaction. Such a reaction would result in the release of enormous amounts of energy in a very brief period of time. They knew that they could build an atomic bomb employing the principle of fission which would produce a very large explosion...