Most nuclear power reactors use Uranium-235 or plutonium-239 as its fuel for generating electricity. When the atomic nuclei of these fuels are struck by neutrons, they can split into smaller nuclei, releasing sizable amounts of energy in the process. Another very important product of this splitting process is the simultaneous release of two or more other neutrons, which can go on to split more fuel nuclei. This becomes a self-perpetuating "chain reaction". This chain reaction perpetuates the desired energy release, and, when properly moderated and controlled, the process can proceed virtually indefinitely, until the original fuel is consumed and diluted to the point where the chain reaction is no longer self-sustaining.