Chain reaction in physics is a process that generates products that cause more processes of the same nature, a self-sustaining sequence. The most typical case of Chain reaction is nuclear fission caused by neutrons. Chain reactions are usually very fast, but are also highly sensitive to reaction conditions, possibly because the substances that support the reaction are easily influenced by substances other than the reactants themselves.
Nuclear chain reactions are a sequence of nuclear fission ( where atomic nuclei splits), each initiated by a neutron released in a previous fission.