Light water reactor

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A light water reactor or LWR is a thermal nuclear reactor where the primary coolant of the reactor core is regular water, as opposed to heavy water reactors such as CANDU that use deuterium oxide as the coolant.

There are three subtypes of LWRs: pressurized water reactors or PWRs (which overlap with heavy water designs), boiling water reactors or BWRs, and supercritical water reactors. All use solid nuclear fuel rods to create heat and ultimately drive a steam turbine, and all require large containment facilities surrounding the reactors in case of pressure failure or coolant release. Additionally, backup systems are required in case of emergency shutdown to stop fission and remove decay heat to prevent meltdowns, and the fuel must be exchanged periodically because of structural damage to the fuel rods caused by xenon gas.

The water coolant also serves as a moderator, or a material that slows neutrons in order to facilitate fission reactions. Light water can also absorb neutrons to help slow the rate of fission. The rate of fission can also be slowed further by adding boric acid to the coolant, as boron also absorbs neutrons.

PWRs are used in naval vessels such as submarines and aircraft carriers, as well as most nuclear power plants.