Pressurized water reactors (PWRs) constitute a majority of all western nuclear power plants and are one of two types of light water reactor (LWR), the other type being boiling water reactors (BWRs). In a PWR the primary coolant (water) is pumped under high pressure to the reactor core, then the heated water transfers thermal energy to a steam 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 light water as both coolant and neutron moderator.
PWRs were originally designed to serve as nuclear submarine power plants and were used in the original design of the second commercial power plant at Shippingport Atomic Power Station.
PWRs currently operating in the United States are considered Generation II reactors. Russia's VVER reactors are similar to U.S. PWRs.
Several hundred PWRs are used for marine propulsion in aircraft carriers, nuclear submarines and ice breakers. In the US, they were originally designed at the Oak Ridge National Laboratory for use as a nuclear submarine power plant. Follow-on work was conducted by Westinghouse Bettis Atomic Power Laboratory. The first commercial nuclear power plant at Shippingport Atomic Power Station was originally designed as a pressurized water reactor, on insistence from Admiral Hyman G. Rickover that a viable commercial plant would include none of the "crazy thermodynamic cycles that everyone else wants to build."
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