Fission refers to the division of a large nucleus into a number of smaller elements (decay products) during nuclear decay. Fission occurs naturally or can be induced by neutron or alpha particle bombardment in nuclear power station reactors or in an atomic weapon.

Fission of large nuclides, such as the various isotopes of uranium and transuranium elements, is accompanied by the release of large amounts of energy. Ionising radiation, such as beta (electrons) and alpha (helium nuclei) particles, and gamma radiation (photons), can also be emitted.

Fission of uranium-235 is the primary reaction exploited in nuclear power generation, although U-238 (to form Pu, plutonium) and Th-233 (thorium, to form U-233) can also be used.

10n + 23592U → 23692U → 14054Xe

+ 9438Sr

+ 210n