The splitting of an atomic nucleus induced by bombardment with neutrons from an external source and propagated by the neutrons so released. When a fissionable (unstable) nucleus, such as 235U or 239Pu, is struck by a neutron in a critical area, the following events occur: (1) the nucleus disintegrates to form several other elements, called fission products or fragments, all of which are radioactive and have high kinetic energy; (2) the disrupted nucleus emits an average of 2.5 neutrons (235U) or three neutrons (239Pu), which in turn split other nuclei of the fissionable material in a chain reaction that is self-perpetuating; (3) it also emits the energy equivalent of the mass defect of the nucleus, usually approximately 200 MeV per nucleus, some of which is in the form of γ rays. A nuclear explosion will not occur until a critical mass of fissionable material is attained, that is, the smallest amount capable of sustaining a chain reaction. Similarly, a nuclear reactor will not produce power until the assembly achieves a critical activity. This occurs as follows: The neutrons introduced to the system are continually escaping--some are lost through the walls, some are captured by structural materials, and some are absorbed by the fissionable atoms themselves without fission taking place. When the neutrons entering the system are very slightly in excess of those lost to it, the assembly is said to be critical and measurable power generation takes place. The ratio is carefully controlled, the rate of energy production rising exponentially. Control rods made of cadmium absorb neutrons so readily that the reactor can function at precisely predetermined levels of activity. Nuclear fission is used for electric power generation and for making radioactive isotopes.See Nuclear Reactor.
