the minimum mass of a sample of fissionable material necessary to sustain a nuclear chain reaction
Nuclear chain reaction Nuclear weapons employ high quality, highly enriched fuel exceeding the critical size and geometry (critical mass) necessary in order to obtain an explosive chain reaction. The fuel for energy purposes, such as in a nuclear fission reactor, is very different, usually consisting of a low-enriched oxide material (e.g. UO2).
Nuclear power In the United States, where Fermi and SzilÃ¡rd had both emigrated, this led to the creation of the first man-made reactor, known as Chicago Pile-1, which achieved criticality on December 2, 1942. This work became part of the Manhattan Project, a massive secret US government military project to make enriched uranium by building large reactors to breed plutonium for use in the first nuclear weapons. The US tested atom bombs and eventually these weapons were used to attack the cities of Hiroshima and Nagasaki.