Light water reactor
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A light water reactor or LWR is a thermal nuclear reactor that uses ordinary water, also called light water, as it's a 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.
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 most common LWRs are pressurized water reactors and boiling water reactors.
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 with 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 are simpler and cheaper than heavy water reactors, and although they have the same power-generating capabilities, it is far more difficult to use them to produce weapons-grade plutonium, as the reactor must be shut down and the fuel rods replaced every four months because if they stay in any longer, the plutonium-240 concentration will become too high and poison the plutonium-239. A disadvantage of light water reactors is that they must use enriched uranium, while heavy water reactors can use natural uranium.
The U.S.–North Korean Agreed Framework as well as the Six-party talks involved the supply of LWRs to North Korea. However, their LWRs have not yet been built and in 2002 North Korea backed out of the agreement.
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