A process in which some of the neutrons released in one fission event cause other fissions to occur. There are three types of chain reactions: (1) Nonsustaining--An average of less than one fission is produced by the neutrons released by each previous fission (reactor subcriticality); (2) Sustaining--An average of exactly one fission is produced by the neutrons released by each previous fission (reactor criticality); and (3) Multiplying--An average of more than one fission is produced by the neutrons released by previous fission (reactor supercriticality).