Pressurised water reactor (PWR)
The most common nuclear reactor type, in which water under high pressure cools the core without boiling and transfers heat to a separate steam circuit.
In a PWR, the primary circuit is held at about 155 bar so water stays liquid at core outlet temperatures of 315–330°C. A pressuriser controls pressure; reactor coolant pumps circulate water through steam generators, where heat passes to a secondary circuit that produces steam at around 60–70 bar for the turbine. Radioactive primary water never reaches the turbine.
PWRs, including Russian VVER designs, account for roughly 70% of the world's operating reactors and capacity. Fuel is uranium dioxide pellets enriched to 3–5% U-235, in zircaloy-clad rods arranged in assemblies; refuelling every 12–24 months replaces about one-third of the core. Water acts as both coolant and moderator, giving a negative void coefficient: if water is lost or boils, reactivity falls, a key inherent safety feature.
Generation III+ designs include the Westinghouse AP1000 (passive cooling for 72 hours without operator action; Vogtle 3 and 4 in the US, Sanmen and Haiyang in China), the EPR (1,600–1,650 MWe; Taishan, Olkiluoto 3, Flamanville 3, Hinkley Point C), the Korean APR-1400, China's Hualong One and Russia's VVER-1200. Typical capacity factors are 80–93%. Most light-water SMRs are scaled-down integral PWRs.
Related terms
All terms →- Boiling water reactor (BWR)
A light-water nuclear reactor in which coolant boils in the core and the resulting steam drives the turbine directly, without steam generators.
- Nuclear fission
The splitting of a heavy atomic nucleus such as uranium-235 or plutonium-239 into lighter nuclei after absorbing a neutron, releasing about 200 MeV of energy and further neutrons.
- Uranium enrichment
The process of increasing the share of fissile U-235 in uranium above its natural 0.711%, to make reactor fuel or, at very high levels, weapons material.
- SMR (Small Modular Reactor)
A nuclear reactor of up to about 300 MWe per module, designed for factory fabrication and modular deployment to cut construction time and financing risk.
- Burnup
The thermal energy extracted from nuclear fuel per unit mass of initial heavy metal, usually expressed in gigawatt-days per tonne (GWd/tHM).
- Capacity uprate
A licensed increase in the maximum power output of an existing power plant, most commonly a nuclear reactor, achieved through better measurement or equipment upgrades.