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.
In a BWR, water enters the core at about 70–75 bar and boils as it rises through the fuel assemblies, leaving at around 285°C with a steam quality of 10–15%. Steam separators and dryers above the core remove moisture, and the steam goes straight to the turbine. This single-loop design removes steam generators and the pressuriser, but the turbine hall becomes a radiation-controlled area because steam carries short-lived activation products such as nitrogen-16.
Varying recirculation flow lets operators change power by about 25% without moving control rods, useful for load following. Control rods are cruciform blades inserted from below. Fuel is uranium dioxide enriched to about 3–5%, often with gadolinium burnable absorbers.
BWRs make up roughly 15% of world reactors, mainly in the United States, Japan, Sweden, Finland, Switzerland, Mexico and Taiwan. Fukushima Daiichi units 1–4 were early GE Mark I BWRs; the 2011 accident, caused by station blackout after the tsunami, led to post-Fukushima upgrades worldwide. Newer designs are the ABWR (1,350 MWe, operating in Japan), ESBWR, with natural circulation and passive safety, and the SMR-scale BWRX-300.
Related terms
All terms →- 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.
- 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.
- 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.
- Load following
Adjusting a power plant’s output over minutes to hours to track changes in electricity demand or in variable renewable generation.
- Spent nuclear fuel
Fuel assemblies removed from a reactor after use, highly radioactive and thermally hot, containing uranium, plutonium, fission products and minor actinides.