Nuclear reactors, also known as atomic PILES, are devices used to initiate and control a sustained nuclear chain reaction. They are used in nuclear power plants to generate electricity, and in nuclear weapons to produce explosive energy. PILES are the most common type of nuclear reactor, consisting of a core of fuel rods surrounded by a moderator and a reflector. The fuel rods contain fissionable material, such as uranium or plutonium, which is split when struck by neutrons. The moderator slows down the neutrons, allowing them to be absorbed by other fuel rods, thus sustaining the chain reaction. The reflector helps to contain the reaction and prevent it from spreading.