Pressurized water reactors (
PWRs) constitute the large majority of all Western
nuclear power plants and are one of three types of
light water reactor (LWR), the other types being
boiling water reactors (BWRs) and
supercritical water reactors (SCWRs). In a PWR, the primary
coolant (
water) is pumped under
high pressure to the reactor core where it is heated by the energy generated by the
fission of atoms. The heated water then flows to a
steam generator where it transfers its thermal energy to a secondary system where steam is generated and flows to turbines which, in turn, spin an electric generator. In contrast to a boiling water reactor, pressure in the primary coolant loop prevents the water from boiling within the reactor. All LWRs use ordinary
water as both coolant and
neutron moderator.