A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reactors are used in nuclear power plants to generate electricity by harnessing the thermal energy released from nuclear fission. The main job of a reactor is to house and control nuclear fission, a process where atoms split and release energy. The energy released from continuous fission of the atoms of the fuel is harnessed as heat in either a gas or water, and is used to produce steam. The steam is used to drive the turbines which produce electricity.
A fission nuclear power plant is generally composed of a nuclear reactor, in which the nuclear reactions generating heat take place; a cooling system, which removes the heat from inside the reactor; a steam turbine, which transforms the heat into mechanical energy; an electric generator, which transforms the mechanical energy into electrical energy. Reactors use uranium for nuclear fuel. The uranium is processed into small ceramic pellets and stacked together into sealed metal tubes called fuel rods. Typically, more than 200 of these rods are bundled together to form a fuel assembly. A reactor core is typically made up of a couple hundred assemblies, depending on power level. Inside the reactor vessel, the fuel rods are immersed in water which acts as both a coolant and a neutron moderator.
Nuclear reactors come in many types, including pressurized water reactors, boiling water reactors, and heavy water reactors. They are also used for research and training, materials testing, or the production of radioisotopes for medicine and industry.
