Pressurized water reactor

The pressurised water reactor (PWR) is the most common type of nuclear reactor, with over 230 in use for power generation and a further several hundred in naval propulsion. The design originated as a nuclear submarine[?] power plant. It uses ordinary water as both coolant and neutron moderator. The design is distinguished by having a primary cooling circuit which flows through the core of the reactor under very high pressure, and a secondary circuit in which steam is generated to drive the turbine.

A PWR has fuel assemblies of 200-300 rods each, arranged vertically in the core, and a large reactor would have about 150-250 fuel assemblies with 80-100 tonnes of uranium. Water in the reactor core reaches about 325°C, hence it must be kept under about 150 times atmospheric pressure to prevent it boiling. Pressure is maintained by steam in a pressuriser. In the primary cooling circuit the water is also the moderator, and if any of it turned to steam the fission reaction would slow down. This negative feedback effect is one of the safety features of the type. The secondary shutdown system involves adding boron to the primary circuit. One disadvantage to this type of reactor is that the reactor continues to generate heat from radioactive decay after the fission reaction is stopped, which can result in a nuclear meltdown if the reactor loses all coolant. As a result, reactors of this type have extensive safety and backup systems to ensure that this does not happen.

The secondary circuit is under less pressure and the water here boils in the heat exchangers which are thus steam generators. The steam drives the turbine to produce electricity, and is then condensed and returned to the heat exchangers in contact with the primary circuit.

A pressured water reactor was involved in the accident at Three Mile Island. Much of the research in civilian nuclear reactors has been to develop reactor designs which survive better after extensive equipment failure.