Reactor families

The moderator slows fission neutrons so that they cause more fission; it is usually ordinary (light) water, heavy water or graphite.

The coolant carries heat out of the core: ordinary water in PWRs and BWRs, heavy water in PHWRs, carbon dioxide in British AGRs, helium in high-temperature gas reactors and liquid sodium in fast reactors.

The IAEA PRIS database listed 417 reactors 'in operation' worldwide with 379,700 MWe net capacity (update of 23 July 2026): 308 PWR, 45 BWR, 46 PHWR, 8 GCR, 7 LWGR, 2 FBR, 1 HTGR.

WNA counts 441 operable power reactors: 316 PWR (306.2 GWe), 60 BWR (61.0 GWe), 47 PHWR (24.5 GWe), 8 AGR (4.7 GWe), 7 LWGR (6.5 GWe), 2 FNR (1.4 GWe) and 1 HTGR (0.2 GWe).

The pressurised water reactor (PWR) is the most common type: ordinary water at over 300 C is kept under about 150 atmospheres in a primary circuit and gives up its heat in steam generators to a separate secondary circuit that drives the turbine.

In a PWR the water in the core reaches about 325 C and is kept under about 150 times atmospheric pressure so that it does not boil; steam is raised in separate steam generators.

Fuel is ceramic UO2 pellets about 1 cm across stacked in zirconium-alloy tubes (fuel rods); a PWR assembly is a 17 x 17 array 4-5 m long, and a 1,100 MWe core holds 193 assemblies, more than 50,000 rods and about 18 million pellets.

A typical PWR fuel assembly is a 17 x 17 square array of fuel rods and guide tubes, 4-5 m long and about 20 cm across, weighing about half a tonne.

Nuclear fission occurs when a uranium-235 nucleus absorbs a neutron and splits into two lighter fragments, releasing about 200 MeV of energy and, on average, 2.45 further neutrons that can sustain a chain reaction.

Control rods or blades made of neutron-absorbing material such as cadmium, hafnium or boron are inserted into or withdrawn from the core to control or halt the chain reaction.

About 0.66% of the neutrons from U-235 fission are 'delayed' - emitted by fission products up to tens of seconds later (the longest-lived precursor group has a half-life of about 56 s) - and this delay is what makes a reactor controllable.

Successive physical barriers hold radioactivity in: ceramic fuel pellets sealed in zirconium-alloy tubes, a steel pressure vessel with walls up to 30 cm thick, and a reinforced-concrete containment with walls at least one metre thick.

A PWR steam generator is a large heat exchanger in which the high-pressure primary coolant makes steam for the turbine in the secondary circuit; reactors have up to six loops, each with a steam generator, and each unit weighs up to 800 tonnes and contains from 300 to 16,000 tubes about 2 cm in diameter carrying the primary coolant.

In a PWR the secondary circuit is under less pressure than the primary circuit and its water 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.

In a PWR the steam line directs the steam to the main turbine, which turns the turbine generator that produces electricity; the unused steam is exhausted to the condenser, condensed into water, pumped out of the condenser by a series of pumps, reheated and pumped back to the steam generator.

The condenser's surplus heat is discharged either by once-through cooling, running a large amount of sea, lake or river water through the condensers once and returning it a few degrees warmer, or by recirculating cooling through a cooling tower, where an updraught of air through water droplets cools the water chiefly by evaporation (natural draught by the chimney effect, or mechanical draught with fans that take about 1% of the plant's output); the tower evaporates up to 5% of the flow.

The containment is the metre-thick concrete-and-steel structure around the reactor and steam generators designed to protect them from outside intrusion and to protect those outside from the effects of radiation in a malfunction.

Any steam-cycle plant loses about two-thirds of its energy in turning heat into mechanical energy; nuclear plants now being built have about 34-36% thermal efficiency depending on the site, older ones often only 32-33%, and a plant at 33% has to dump about 14% more heat than one at 36%.

The boiling water reactor (BWR) has a single circuit in which water at about 75 atmospheres boils in the core (about 285 C) and the steam passes directly to the turbine.

Pressurised heavy water reactors (CANDU and Indian PHWRs) use natural uranium oxide fuel with heavy water as moderator in a pressure-tube design, which lets them be refuelled progressively without shutting down.

CANDU, RBMK and AGR reactors can be refuelled under load; light-water reactors must shut down to refuel.

VVER is the Russian family of water-moderated and water-cooled pressurised reactors (the VVER-1200 is the current Generation III+ model with about 1,085-1,114 MWe net).

Britain's advanced gas-cooled reactors (AGRs) use a graphite moderator and carbon dioxide coolant with fuel enriched to 2.5-3.5%; eight remain operable.

The Soviet RBMK (light-water graphite reactor) has vertical pressure tubes running through a graphite moderator in which the water is allowed to boil; seven remain in operation.

High-temperature gas-cooled reactors (HTGRs) use helium coolant, a graphite moderator and TRISO coated-particle fuel, reaching about 750-950 C; the only commercial unit is China's HTR-PM (two 250 MWt pebble-bed modules on one 210 MWe turbine, commercial operation declared 6 December 2023).

TRISO fuel particles for high-temperature gas reactors are less than 1 mm across, with a uranium kernel of about 0.5 mm enriched up to 20%, and run at 750-950 C.

Fast neutron reactors have no moderator and use a liquid-metal coolant (normally sodium) at around 500-550 C and near-atmospheric pressure; they can breed plutonium from U-238 or burn long-lived actinides, extracting about 60 times more energy from uranium than a conventional reactor.

About 20 fast reactors have operated, accumulating over 400 reactor-years; the two operating commercial units are Russia's BN-600 (since 1980) and BN-800 (since 2014, full MOX core from September 2022) at Beloyarsk, while China's CEFR went critical in July 2010 and India's PFBR began core loading in March 2024.

Molten salt reactors use molten fluoride (or chloride) salts as coolant and usually as the fuel carrier, at low pressure and about 700 C; freeze plugs can drain the salt into dump tanks on overheating.

The only MSR to have operated is the 8 MWt Molten Salt Reactor Experiment at Oak Ridge (1965-69, with a U-233 campaign in 1968-69); China's 2 MWt TMSR-LF1 at Wuwei, Gansu, was listed by WNA as under construction on its page of November 2025.