A chain reaction is also called nuclear fission chain reaction. A chain reaction is a reaction which is a combination of continuous nuclear fission reactions because the nuclear reaction resulting from the capture of a neutron is accompanied by emission of one or more neutrons. These neutrons which are released cause further fission. For example- the fission reaction of U235 by bombarding with neutron results into formation of three more neutrons. These three neutrons further get bombard with U235 atoms to perform nuclear fission process, Thereby setting up a chain reaction. Hence, once nuclear fission process started, it would be self sustaining accompanied by continues release of energy.
92U235 + 01n ——> 56Ba141 + 36Kr94 + 3 01n + Enormous energy
