A nuclear reactor initiates and controls a fission chain reaction to produce neutrons and/or energy. Nuclear fission is a process where the nucleus of an atom is split into two or more smaller nuclei, known as fission products. Fission of heavy elements (elements with an atomic number greater than 92) results in a reaction that releases large amounts of heat energy. Nuclear reactors are used as research tools, as systems for producing radioactive isotopes, and as energy sources for nuclear power plants. When used in power plants, the fusion created by the reactor provides the heat to create steam for a steam turbine.
Diagram of a nuclear reactor in a pressurized water reactor power plant
Source: World Nuclear Association
A key factor in operating nuclear power plants is controlling the fission to prevent excess heat. The rate of the nuclear reaction is controlled by moving fuel rods deeper into or further out of the reactor core, thus controlling the amount of heat released. A nuclear reactor coolant, usually water but sometimes a gas or a liquid metal, is circulated past the reactor core to absorb the heat that it generates. The heat is carried away from the reactor and is then used to generate steam. If control of the nuclear reaction is lost, it is possible for a nuclear reactor to melt down and release radioactive nuclear material into the environment. For this reason, emergency cooling systems and containment vessels are important components of a nuclear power plant. In the U.S., safety of nuclear reactors is regulated by the Nuclear Regulatory Commission (NRC).