![]() ![]() In the case of a nuclear reactor, the number of fissionable nuclei available in each generation is carefully controlled to prevent a “runaway” chain reaction. If the number of fissions in one generation is equal to the number of neutrons in the preceding generation, the system is said to be critical if the number is greater than one, it is supercritical and if it is less than one, it is subcritical. The continuing process whereby neutrons emitted by fissioning nuclei induce fissions in other fissile or fissionable nuclei is called a fission chain reaction. ![]() Many heavy atomic nuclei are capable of fissioning, but only a fraction of these are fissile-that is, fissionable not only by fast (highly energetic) neutrons but also by slow neutrons. A portion of these neutrons are captured by nuclei that do not fission others escape the material without being captured and the remainder cause further fissions. In an atomic bomb or nuclear reactor, first a small number of neutrons are given enough energy to collide with some fissionable nuclei, which in turn produce additional free neutrons. As a rule of thumb, the complete fission of 1 kg (2.2 pounds) of uranium or plutonium produces about 17.5 kilotons of TNT-equivalent explosive energy. In addition to this formation of lighter atoms, on average between 2.5 and 3 free neutrons are emitted in the fission process, along with considerable energy. When bombarded by neutrons, certain isotopes of uranium and plutonium (and some other heavier elements) will split into atoms of lighter elements, a process known as nuclear fission.
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