Nuclear fission is when an atomic nucleus splits into two or more pieces creating the energy for nuclear power plants or weaponry. This splitting is often triggered by the absorption of a neutron travelling at high speed. Uranium-235 and Plutonium-239 are readily fissile and can release lots of energy. When they split they will release more neutrons, a property that allows for neutron chain reactions. Since Plutonium-239 tends to occur artificially from Uranium-235 it is less common.

Neutron chain reactions are required to produce sufficient energy for nuclear weaponry and power. When a readily fissile material (e.g. Uranium-235, Plutonium-239) undergoes fission it produces more neutrons. The neutrons produced are supposed to cause more atoms to undergo fission. With enough of these atoms present enough energy can be produced to power nuclear reactors and even weapons.