Fission

Fission is the splitting of an atomic nucleus splits into several smaller fragments of roughly half the original mass. The process emits two or three neutrons.

The sum of the fragments is slightly less than than the original mass, and the "missing" mass constitutes about 0.1 percent of the original mass. This "missing" mass is converted into energy.

Fission occurs when a heavy atom nucleus captures a neutron. Fission can also happen spontaneously in nature.