Light water reactor

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Core of CROCUS, a small light water nuclear reactor used for research at the EPFL in Switzerland.

A light water reactor or LWR is a thermal nuclear reactor that uses ordinary water, also called light water, as its neutron moderator. This differentiates it from a heavy water reactor, which uses heavy water as a neutron moderator. In practice all LWRs are also water cooled. While ordinary water has some heavy water molecules in it, it is not enough to be important in most applications.

The most common LWR are pressurized water reactors and boiling water reactors.

The Russian abbreviation for LWR is VVR (or sometimes WWR), meaning water water reactor. Similarly, the Russian term for a PWR is VVER, meaning water water energy reactor.

Many other reactors are also (light) water cooled, notably the RBMK and some military plutonium production reactors. These are not regarded as LWRs, as they are moderated by graphite, and as a result their nuclear characteristics are very different.

The light-water reactor uses uranium 235 as a fuel, enriched to approximately 3 percent. Although this is its major fuel, the uranium 238 atoms also contribute to the fission process by converting to plutonium 239 — about one-half of which is consumed in the reactor. Light-water reactors are generally refueled every 12 to 18 months, at which time, about 25 percent of the fuel is replaced.

Light water reactors tend to be simpler and cheaper to build than heavy water reactors. Power-generating capabilities are comparable.

Light water reactors are the type used by the U.S. military in its Naval nuclear powered vessels. This is so due to the inherent safety of these type reactors. Since light water is used as both a coolant and a moderator in these reactors, if one of these reactors suffers damage due to attack, and thereby compromise of the reactor core's integrity, the ensuing release of this light water acts to shut down the reactor. Moderators help to encourage the nuclear mass to achieve fission by lowering the average speed of the neutrons to a level which augments the probability of occurrence of neutron-Uranium collisions susceptible to lead to a Uranium nucleus fission. When the moderator is removed, the average energy of the neutrons becomes too high for the chain reaction to sustain itself. Currently-offered LWRs include the ABWR, AP1000, ESBWR, European Pressurized Reactor, VVER and SWR-1000.