There are nuclei that can undergo fission
on their own spontaneously, but only certain nuclei, like uranium-235
, can sustain a fission chain reaction
. This is because these nuclei release neutrons
when they break apart, and these neutrons can induce fission of other nuclei. Free neutrons released by each fission play very important role as a trigger of the reaction.
Nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts (lighter nuclei). This nuclear reaction is triggered by the neutron. Source: chemwiki.ucdavis.edu