Definition - What does Nuclear Fission mean?
Nuclear fission occurs when the nucleus of an atom splits in two. The reaction results in two smaller fragments termed fission products, and two to three neutrons are released. The combined mass of the fission products is less than the mass of the original. The missing mass has been converted into energy.
Safeopedia explains Nuclear Fission
Fission can occur naturally in atoms with large nuclei. It can also be caused by bombarding fuel with neutrons. The energy produced by fission is used in power reactors that generate energy, in reactors used for research purposes and in breeder reactors that are used to produce nuclear fuels. In critical fission reactors, the neutrons that are released during fission are used to cause further fissions, thereby becoming self-sustaining. Sub-critical fission reactors engineer fission, but do not sustain the reaction.