Fission reactors are divided into thermal and fast reactors according to the kinetic energy of the neutrons. Thermal reactors are divided into various reactor types according to the fuel, coolant, and moderator.
There are seven major power plant reactor types currently in commercial use worldwide. The most common of these is the pressurized water reactor (PWR). More than 60 percent of all reactors are pressurized water reactors. The second most common type is the boiling water reactor (BWR), accounting for one in five of all reactors. Other reactor types currently in use are the heavy water reactor (HWR), the gas cooled reactor (Magnox), the advanced gas cooled reactor (AGR) with graphite moderator, the Soviet-designed high-power channel reactor (RBMK) with graphite moderator, and the fast breeder reactor (FBR).
A great majority of pressurized and boiling water reactors are light water reactors. This means that they use regular purified water as the coolant and moderator. The moderator in heavy water reactors is heavy water in which the water molecules contain not hydrogen but deuterium, a heavy hydrogen isotope. There are also graphite-moderated reactors and gas-cooled reactors. Possible cooling gases include carbon dioxide and helium. Nearly all nuclear reactors currently in operation are based on slow neutrons, but a few plants exist where the reaction is based on fast neutrons. Fast reactors are generally called fast breeder reactors, because they are able to transmute a non-fissile uranium isotope, uranium-238, into plutonium-239, which can then be used as nuclear fuel.
Read more about neutrons