A nuclear reactor is a device that initiates and controls a sustained nuclear chain reaction. It is primarily used for generating electricity, but can also produce isotopes for medical and industrial applications. The reactor contains fuel, usually made of uranium or plutonium, which undergoes fission, releasing a large amount of energy in the form of heat. This heat is used to produce steam, which drives turbines connected to generators, converting thermal energy into electrical energy. Safety systems are in place to manage the reactor's operation and prevent accidents. The design and operation of nuclear reactors are regulated by organizations like the Nuclear Regulatory Commission (NRC) in the United States.