High Quality Content by WIKIPEDIA articles! The light water reactor (LWR) is a type of thermal reactor that uses normal water as its coolant and neutron moderator and a solid compound of fissile element as its fuel. Thermal reactors are the most common type of nuclear reactor, and light water reactors are the most common type of thermal reactor. There are three varieties of light water reactors: the pressurized water reactor (PWR), the boiling water reactor (BWR), and (most designs of) the supercritical water reactor (SCWR).