nuclear reactor

nuclear reactor Device in which nuclear fission (and sometimes nuclear fusion) reactions are used for power generation or for the production of radioactive materials. In the reactor, the fuel is a radioactive heavy metal: uranium-235, uranium-233, or plutonium-239. In these metals, atoms break down spontaneously, undergoing a process called radioactive decay. Some neutrons released in this process strike the nuclei of fuel atoms, causing them to undergo fission and emit more neutrons. These, in turn, strike more nuclei, and in this way a chain reaction is set up. Usually a material, called a moderator, is used to slow down the neutrons to a speed at which the chain reaction is self-sustaining. This process occurs within the reactor core. The chain reaction is regulated by inserting control rods, which contain neutron-absorbing material such as cadmium or boron, into the core. The heat generated by the nuclear reaction is absorbed by a circulating coolant, and transferred to a boiler to raise steam. The steam drives a turbine that turns a generator, that in turn produces electricity. There are a variety of nuclear reactors, named after the type of coolant they use. For example, a boiling-water reactor (BWR) and a pressurized-water reactor (PWR), presently the most common type of reactor, both use water as the coolant and the moderator. In advanced gas-cooled reactors (AGR), the coolant is a gas – most commonly carbon dioxide. Fast reactors do not use a moderator, and fission is caused by fast neutrons. This type of reactor generates greater temperatures, and the coolant used is a liquid metal (usually liquid sodium). Sometimes called ‘fast-breeder’ reactors, fast reactors produce (‘breed’) more fissionable material than they consume. Excess neutrons from the fission of a fuel such as Ur²³⁵, instead of being absorbed in control rods, are used to bombard atoms of relatively inactive Ur²³⁸ which transmutes to the active isotope Pu²³⁹. When the original fuel is spent, the plutonium can be used as a nuclear fuel in other reactors or nuclear weapons. See also electricity sources; transmutation