The process in which a massive atomic nucleus breaks apart into a number of fragments. The protons and neutrons in the fragments will be bound more tightly than they were in the original nucleus, so the total mass of the fragments will be less than that of the original nucleus, the loss of mass being released as energy (E) according to the Einstein equation E = mc2, where m is the lost mass and c is the speed of light. In a nuclear reactor, a uranium-235 atomic nucleus captures a low-energy (‘slow’) neutron, rendering it unstable, and it undergoes fission with the release of more neutrons and more than 200 million times the energy of the neutron that caused the fission.