A facility in which fissile material is used in self-sustaining chain reactions (nuclear fission) to produce heat and/or radiation for both practical application and research and development. A reactor includes fissionable material (fuel) such as uranium or plutonium, and a moderating material and usually includes a reflector to conserve escaping neutrons, provisions for heat removal, and measuring and control elements.

See Fission, Boiling-Water Reactor, Light-Water Reactor, Pressurized-Water Reactor.