A self-sustaining nuclear reaction; a series of nuclear fissions in which neutrons released by splitting one atom leads to the splitting of others WordNet 3.0 & Open English WordNet

In nuclear physics, a nuclear chain reaction occurs when one single nuclear reaction causes an average of one or more subsequent nuclear reactions, thus leading to the possibility of a self-propagating series or "positive feedback loop" of these reactions. Wikipedia

One single nuclear reaction causes more subsequent nuclear reactions Wikidata

(physics) A nuclear reaction in which particles produced by the fission of one atom trigger fissions of other atoms. OmegaWiki

A nuclear reaction in which particles produced by the fission of one atom trigger fissions of other atoms. Wiktionary