See also: Neutron cross-section. Comparison of total fission cross-sections. Uranium Comparison of total fission cross-section and cross-section for radiative capture. Neutron Nuclear Reactions. Nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts lighter nuclei. The fission process often produces free neutrons and photons in the form of gamma raysand releases a large amount of energy. In nuclear physics, nuclear fission is either a nuclear reaction or a radioactive decay process.