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The light-water reactor (LWR) is a type of thermal-neutron reactor that uses normal water, as opposed to heavy water, as both its coolant and neutron moderator; furthermore a solid form of fissile elements is used as fuel. Thermal-neutron reactors are the most common type of nuclear reactor, and light-water reactors are the most common type of ...
A graphite-moderated reactor is a nuclear reactor that uses carbon as a neutron moderator, which allows natural uranium to be used as nuclear fuel. The first artificial nuclear reactor, the Chicago Pile-1 , used nuclear graphite as a moderator.
Water (sometimes called "light water" in this context) is the most commonly used moderator (roughly 75% of the world's reactors). Solid graphite (20% of reactors) and heavy water (5% of reactors) are the main alternatives. [1] Beryllium has also been used in some experimental types, and hydrocarbons have been suggested as another possibility.
As graphite already moderates neutrons, light water has a lesser effect in slowing them down, but could still absorb them. This means that the reactor's reactivity (adjustable by appropriate neutron-absorbing rods) must take into account the neutrons absorbed by light water.
Nuclear reactors with water moderator require at least some enrichment of 235 U. Nuclear reactors with heavy water or graphite moderation can operate with natural uranium, eliminating altogether the need for enrichment and preventing the fuel from being useful for nuclear weapons; the CANDU power reactors used in Canadian power plants are an ...
Light water graphite reactors were built only in the USSR. RBMK designs used secondary containment-like structures, but the reactor's top plate was a part of the protective structure. During the Chernobyl accident in 1986 the plate suffered a pressure beyond the predicted limits and lifted up.
As the RBMK, the EGP-6 uses water for cooling and graphite as a neutron moderator. EGP is a Russian acronym but translated into English it stands for Power Heterogenous Loop reactor. [1] It is the world's smallest running commercial nuclear reactor, however smaller reactors are currently in development. [2]
Almost all currently operating nuclear power plants are light water reactors using ordinary water under high pressure as coolant and neutron moderator. About 1/3 are boiling water reactors where the primary coolant undergoes phase transition to steam inside the reactor. About 2/3 are pressurized water reactors at even higher