Chain reaction is a process in which a mass of uranium or plutonium releases nuclear energy. That energy is created by a succession of nuclear fissions that continues automatically once it has been started. A fission is a splitting of an atomic nucleus.
Chain reactions produce energy in a nuclear reactor. In a reactor that uses uranium fuel, the nucleus of a uranium 235 atom captures an uncharged subatomic particle called a neutron. Uranium 235 is the isotope (form) of uranium whose nucleus contains 92 positively charged particles called protons and 141 neutrons. After capturing the extra neutron, the uranium 235 nucleus splits into two smaller nuclei called fission fragments, releasing energy and several neutrons. Those neutrons strike other uranium 235 nuclei and cause them to break apart. Those splittings, in turn, give off additional neutrons, which split still more nuclei. The chain reaction can cause trillions of atoms to fission within a fraction of a second.
See also Nuclear reactor ; Nuclear weapon (Fission weapons) .
