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. Waste heat from small PWRs has also been used for heating in polar regions.
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.
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.
Many PWRs have a secondary shutdown system which involves injecting boron, a strong neutron absorber, into the primary circuit. Boron is also 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.
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, condenses into water and returns to the heat exchangers to be heated again.
A pressured 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.