An off-load refuelled power reactor which is both moderated and cooled by ordinary (light) water. LWR fuel assemblies usually consist of Zircaloy clad fuel rods containing uranium oxide pellets of low enrichment, generally less than 5% ²³⁵U, or mixed oxide (MOX) having a low plutonium content, generally less than 5%. There are two types of LWR: boiling water reactors (BWRs) and pressurized water reactors (PWRs). In a BWR, the heat generated is extracted by allowing the water to boil as it passes through the reactor core, the steam raised being passed directly to the turbine. In a PWR, the reactor vessel is operated at a pressure sufficient to suppress the boiling of the water; the steam required for the turbine is produced in the secondary circuit by passing the primary coolant water through heat exchangers (steam generators).