"Spontaneous fission" refers to the breaking apart or splitting of an atomic nucleus without any external influence or input. This process happens spontaneously, or on its own, and often releases a significant amount of energy. Full definition
Because of the way Los Alamos was run, Robert Oppenheimer could respond to the crisis over spontaneous fission by rapidly redeploying his staff into developing the technically highly complex «implosion'to achieve a critical mass of plutonium.) (newscientist.com)
But they do not stop spontaneous fissions in fuel rods from fuel and fission products. (sciencemag.org)
Uranium - 235 undergoes spontaneous fission during radioactive decay; however, no standard equation can represent this reaction as its results are quite unpredictable. (chemistrylearner.com)