Nuclear Fission

Nuclear fission occurs when a heavy nucleus (such as uranium-235 or plutonium-239) absorbs a neutron and splits into two roughly equal smaller nuclei, releasing two or three neutrons and a large amount of energy.

The released neutrons can cause further fission events, creating a chain reaction. In a nuclear reactor, control rods absorb excess neutrons to maintain a steady rate of fission.

The energy released comes from the mass difference between the reactants and products ($E = mc^2$).