chain re·ac·tion • n. a chemical reaction or other process in which the products themselves promote or spread the reaction, which under certain conditions may accelerate dramatically. ∎  the self-sustaining fission reaction spread by neutrons that occurs in nuclear reactors and bombs. ∎  fig. a series of events, each caused by the previous one: an article in one publication sets off a chain reaction in the media.

Chain reaction

A situation in which one action causes or initiates a similar action. In a nuclear chain reaction, for example, a neutron strikes a uranium-235 nucleus, causing the nucleus to undergo fission, which in turn produces a variety of products. Among these products is one or more neutrons. Thus, the particle needed to initiate this reaction (the neutron) is itself produced as a result of the reaction. Once begun, the reaction continues as long as uranium-235 nuclei are available. Nuclear chain reactions are important sources of fission and fusion energy.

chain reaction Self-sustaining nuclear fission reaction (splitting an atomic nucleus) in which one reaction is the cause of a second, the second of a third and so on. The initial conditions are critical, as the quantity of fissionable material must exceed the critical mass, which is the mimimum mass needed of fissionable material. The explosion of an atom bomb is an uncontrolled chain reaction.