A similar process occurs in fissionable isotopes (such as uranium-238), but in order to fission, these isotopes require additional energy provided by fast neutrons (such as those produced by nuclear fusion in thermonuclear weapons).
En lignende proces sker i fissionerbare isotoper (som uran-238), men for at fissionere, kræver disse isotoper yderligere energi som leveres af hurtige neutroner. (som f.eks. dem der frembringes ved kernefusion i termonukleare våben).