<P> Fission chain reactions occur because of interactions between neutrons and fissile isotopes (such as U). The chain reaction requires both the release of neutrons from fissile isotopes undergoing nuclear fission and the subsequent absorption of some of these neutrons in fissile isotopes . When an atom undergoes nuclear fission, a few neutrons (the exact number depends on several factors) are ejected from the reaction . These free neutrons will then interact with the surrounding medium, and if more fissile fuel is present, some may be absorbed and cause more fissions . Thus, the cycle repeats to give a reaction that is self - sustaining . </P> <P> Nuclear power plants operate by precisely controlling the rate at which nuclear reactions occur, and that control is maintained through the use of several redundant layers of safety measures . Moreover, the materials in a nuclear reactor core and the uranium enrichment level make a nuclear explosion impossible, even if all safety measures failed . On the other hand, nuclear weapons are specifically engineered to produce a reaction that is so fast and intense it cannot be controlled after it has started . When properly designed, this uncontrolled reaction can lead to an explosive energy release . </P> <P> 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. UO). </P> <P> When a heavy atom undergoes nuclear fission, it breaks into two or more fission fragments . Also, several free neutrons, gamma rays, and neutrinos are emitted, and a large amount of energy is released . The sum of the rest masses of the fission fragments and ejected neutrons is less than the sum of the rest masses of the original atom and incident neutron (of course the fission fragments are not at rest). The mass difference is accounted for in the release of energy according to the equation E = Δmc: </P>

The minimum mass of a sample of fissionable material necessary to sustain a nuclear chain reaction