A pressurized water reactor (PWR) is a type of nuclear power reactor that uses ordinary (light) water for both coolant and for neutron moderator.

In a PWR, the primary coolant loop is pressurised so the water does not boil, and heat exchangers called steam generators are used to transmit heat to a secondary coolant which is allowed to boil to produce steam either for warship propulsion or for electricity generation. In having this secondary loop the PWR differs from the boiling water reactor (BWR), in which the primary coolant is allowed to boil in the reactor core and drive a turbine directly. Heat from small PWRs has also been used for heating in polar regions, see Army Nuclear Power Program.

This is the most common type of nuclear power reactor. More than 230 are in use to generate electric power, and several hundred more for naval propulsion. The design originated as a nuclear submarine power plant.

• Coolant. Neutrons striking nuclear material (mainly U-235) in fuel rods lead to fissioning of the atoms, releasing more neutrons and heat. The heat passes through the fuel rod and its metal "cladding" into water flowing in channels along the length of the fuel assembly. That water flows to a steam generator. There, the heat passes to water in a secondary circuit that becomes steam for use outside the reactor enclosure.

• Moderator. Nuclear fission produces neutrons that are too hot to trigger significant further fission within the reactor fuel. Their energy must first come down to so-called "thermal" levels in rough equilibrium with the temperature of the surrounding medium, which might be 450°C (800°Fahrenheit). In the PWR, these neutrons initially lose heat when they collide with molecules of coolant water. After several collisions (8 - 10 on average), a neutron reaches the temperature of its surroundings and is likely to be absorbed by a Uranium-235 atom. Such absorption leads quickly to fission of the Uranium atom.

A typical PWR has fuel assemblies of 200-300 rods each, and a large reactor would have about 150-250 such assemblies with 80-100 tonnes of uranium in all. It produces electric power in the order of 900 to 1500 MW.

Rancho Seco PWR reactor hall and cooling tower (being decommissioned, 2004)

A key mechanism that controls any nuclear reactor is the rate at which fission events release neutrons. On average, each fission releases just over two neutrons with a lot of heat. When a neutron strikes a Uranium atom a further fission event can occur, and this can lead to a chain reaction. If all neutrons were released instantaneously, their number would grow very fast, resulting in the destruction of the fuel cells and a melt-down of the reactor. However, a small fraction of these neutrons are released over an extended period (perhaps one minute). This small, but crucial, delayed release permits the other control mechanisms (negative temperature co-efficient, human or computer manipulation of neutron-absorbing control rods, etc.) to have an effect.

Water in a PWR reactor core reaches about 325°C, only remaining liquid under about 150 times atmospheric pressure to prevent it boiling. Pressure is maintained by steam in a pressuriser. In the reactor core, the primary cooling circuit water is also the moderator, and if any of it turned to steam the fission reaction would slow down. This negative feedback effect is called a negative void coefficient and is one of the safety features of the PWR.

Another advantage of using coolant water as a moderator in a pressurized water reactor is that the moderating effect decreases as a function of temperature due to the negative temperature coefficient of reactivity. This results in a stabilizing effect where increases in temperature cause decreases in reactor power, while decreases in temperature cause increases in reactor power. This acts as a negative feedback loop, ensuring reactor power is the minimum required to supply the heat being drawn off by the secondary steam system. A disadvantage is that the reactor is succeptible to produce power at rates that result in damage to fuel cells in the event of introduction of cold water into the reactor or in the event the secondary system experiences a steam rupture.

The secondary circuit is under less pressure than the primary. The secondary water boils in heat exchangers which generate steam. The steam drives the turbine to produce electricity or turn a drive shaft of a ship. This steam then condenses into water and returns to the heat exchangers to be heated again.

Many PWRs have a secondary shutdown system which involves injecting boron, a strong neutron absorber, into the primary circuit. Boron is also sometimes added to the primary cooling water during normal operation to allow more highly enriched fuel to be used, prolonging the fuel life. A drawback is that this makes the cooling water corrosive.

One disadvantage to this type of reactor is that radioactive decay continues to generate significant heat even after the fission reaction stops, possibly leading to nuclear meltdown if the reactor loses all coolant. PWR's typically have extensive safety and backup systems to prevent this. However, the complexity of these systems has been criticized on the grounds that in an emergency, they may be prone to unexpected interactions and operator error.

A pressurized water reactor was involved in the accident at Three Mile Island. Much of the research in civilian nuclear reactors has been targeted to improve their resilience even after extensive equipment failure.