A reaction that is self-sustaining as a result of the products of one step initiating a subsequent step.
In nuclear chain reactions the succession depends on production and capture of neutrons. Thus, one nucleus of the isotope uranium–235 can disintegrate with the production of two or three neutrons, which cause similar fission of adjacent nuclei. These in turn produce more neutrons. If the total amount of material exceeds a critical mass, the chain reaction may cause an explosion.
Chemical chain reactions usually involve free radicals as intermediates. An example is the reaction of chlorine with hydrogen initiated by ultraviolet radiation. A chlorine molecule is first split into atoms:
These react with hydrogen as follows
Combustion and explosion reactions involve similar free-radical chain reactions.