Reactor types explained

Reactor names are a thicket of acronyms, but almost every design can be placed by answering three questions. This guide sets out those questions, then takes each major reactor type in turn: how it works, where it runs or has run, its main advantage, its main open problem, and which companies in this dataset are building one.

The how fission reactors work guide explains the terms used here: moderator, coolant, thermal and fast neutrons.

Three questions

1. What carries the heat away? This is the coolant: ordinary water, heavy water, helium, liquid sodium, liquid lead, molten salt, or sealed heat pipes. The coolant sets the operating pressure and temperature, and so much of the engineering. Water must be kept at high pressure to stay liquid at useful temperatures. Sodium, lead and salt stay liquid at high temperature near atmospheric pressure.

2. Are the neutrons slowed down? If a moderator is present (water, heavy water or graphite), the reactor is thermal. If not, it is fast. The spectrum decides how enriched the fuel must be, what the fuel can burn or breed, and how hard the neutrons are on the structure.

3. What form is the fuel in? Most reactors use solid ceramic pellets of uranium oxide in metal tubes. Others use metal fuel, or TRISO particles (grains of fuel sealed in carbon and silicon carbide coatings). One family dissolves the fuel in a molten salt, so the fuel itself flows around the circuit.

The third question is where classification most often goes wrong. Coolant is not fuel form. A reactor cooled by molten salt with solid TRISO fuel is a different machine from one whose fuel is dissolved in the salt. The first keeps its fission products locked in solid particles inside the core; the second carries them through pumps and heat exchangers. This dataset holds the two apart: salt-cooled and molten-salt are separate approaches.

The fleet today

The World Nuclear Association counted the following operable power reactors on 5 August 2026:

Type	Units	Capacity (GWe)
Pressurised water reactor (PWR)	316	306.2
Boiling water reactor (BWR)	60	61.0
Pressurised heavy-water reactor (PHWR)	47	24.5
Advanced gas-cooled reactor (AGR, UK)	8	4.7
Light-water graphite reactor (RBMK type)	7	6.5
Fast neutron reactor	2	1.4
High-temperature gas-cooled reactor	1	0.2

About 85% of units are light-water reactors (PWR and BWR: 376 of 441). Everything else below is either a small share of the fleet or not yet operating as a power plant.

Pressurised water reactor (PWR)

How it works. Ordinary water is both coolant and moderator. It is held at high pressure so it does not boil: the Rolls-Royce SMR design, for example, specifies 15.5 MPa in the primary circuit, about 150 times atmospheric pressure, with coolant at 295 to 322 °C. The hot water passes through a steam generator, where it boils water in a separate circuit that drives the turbine. Fuel is uranium oxide, usually enriched to below 5% uranium-235.

Where it runs. 316 units worldwide, the most common type by far.

Main advantage. Experience. Fuel supply, licensing rules, supply chains and trained staff for it exist in most countries that run nuclear power.

Main open problem. The engineering burden of high pressure: thick forged pressure vessels, and a design basis built around losing that pressure in a pipe break. Small PWR designs try to reduce this by putting the steam generators inside the vessel and removing large pipes.

In this dataset. Westinghouse, Framatome, NuScale Power, Holtec International, Rolls-Royce SMR, Last Energy, Deep Fission, Hadron Energy, NUWARD, EDF, CGN, CNNC, State Power Investment Corporation, Rosatom and its design bureau OKBM Afrikantov, Korea Hydro & Nuclear Power, KEPCO Engineering & Construction, KAERI and CNEA, among others. See PWR.

Boiling water reactor (BWR)

How it works. Light water is again both coolant and moderator, but it boils in the core at lower pressure (the BWRX-300 design specifies 7.17 MPa) and the steam goes straight to the turbine. There is one circuit instead of two.

Where it runs. 60 units.

Main advantage. No steam generators, fewer large components.

Main open problem. Steam that has passed through the core carries short-lived radioactivity, so the turbine building must be shielded and access to it restricted during operation.

In this dataset. GE Vernova Hitachi, whose 300 MWe BWRX-300 received a Canadian licence to construct at Ontario Power Generation’s Darlington site on 4 April 2025. Hitachi GE Vernova Nuclear Energy, a separate company based in Japan, is also filed under BWR, as are the utilities Teollisuuden Voima and Orlen Synthos Green Energy. The Tennessee Valley Authority’s construction permit application for a BWRX-300 at Clinch River was accepted for review by the US Nuclear Regulatory Commission on 9 July 2025. See BWR and licensing.

Pressurised heavy-water reactor (PHWR, CANDU)

How it works. Heavy water, in which hydrogen is replaced by deuterium, is used as moderator and coolant. Heavy water absorbs far fewer neutrons than ordinary water, so the reactor can run on natural uranium (0.7% uranium-235) with no enrichment. The fuel sits in many horizontal pressure tubes rather than one large vessel, which allows refuelling one channel at a time while the reactor keeps running.

Where it runs. 47 units. The World Nuclear Association says the design has been developed in Canada as CANDU since the 1950s, and also in India from the 1980s.

Main advantage. No enrichment plant needed, and no shutdowns for refuelling.

Main open problem. Heavy water is expensive to produce, and the design has a small positive void coefficient: if coolant boils, reactivity rises slightly, and the control systems must respond. It is small enough for them to handle, but it is the opposite sign to a light-water reactor.

In this dataset. CANDU designer AtkinsRéalis, and operators Bruce Power, Ontario Power Generation, NB Power, NPCIL, Nuclearelectrica and Nucleoeléctrica Argentina. See PHWR.

High-temperature gas-cooled reactor (HTGR)

How it works. Helium gas carries the heat and graphite moderates the neutrons. Fuel is TRISO particles, packed into billiard-ball-sized pebbles or into cylindrical compacts. The X-energy Xe-100 design, for example, heats helium from 260 °C to 750 °C.

Where it has run. Peach Bottom (1967 to 1974) and Fort St Vrain (1976 to 1989) in the United States, AVR (1967 to 1988) and THTR (1983 to 1989) in Germany. Today Japan’s HTTR (30 MW thermal) and China’s HTR-10 are research reactors, and China’s HTR-PM at Shidao Bay is the one power plant: two 250 MW thermal reactor modules driving one 210 MWe turbine, connected to the grid on 20 December 2021.

Main advantage. High outlet temperature, which suits industrial heat as well as electricity, and fuel particles that came through more than 300 hours at temperatures up to 1,800 °C in tests with no to minimal damage.

Main open problem. Fuel supply. Most new designs use TRISO made with high-assay low-enriched uranium (HALEU, 5 to 20% uranium-235), which the US Department of Energy says “isn’t yet available at a commercial scale”. See nuclear fuel explained.

In this dataset. X-energy, BWX Technologies, Radiant, Valar Atomics, Jimmy, NANO Nuclear Energy, Rosatom and OKBM Afrikantov, Japan’s JAEA, Tsinghua University’s Institute of Nuclear and New Energy Technology, the operator China Huaneng Group and fuel maker TRISO-X. Terra Innovatum is classed as gas-cooled without being assigned to HTGR; its record describes the SOLO design as fuelled with conventional low-enriched uranium oxide pellets. See HTGR.

Fluoride salt-cooled high-temperature reactor (FHR)

How it works. Solid TRISO fuel, as in an HTGR, but cooled by a molten fluoride salt instead of helium. Kairos Power’s KP-FHR uses Flibe, a mix of lithium fluoride and beryllium fluoride, at under 0.2 MPa, with the salt heated from 550 °C to 650 °C and fuel enriched to 19.75%.

Where it runs. No FHR has yet operated. Kairos received a US construction permit for Hermes, a 35 MW thermal non-power test reactor at Oak Ridge, Tennessee, on 14 December 2023. Construction permits for two Hermes 2 units followed on 21 November 2024.

Main advantage. Near-atmospheric pressure and a coolant whose heat capacity per unit volume is similar to water’s, while keeping the robust solid fuel.

Main open problem. Salt chemistry and supply. Flibe for a reactor needs lithium almost entirely of the isotope lithium-7, because lithium-6 absorbs neutrons and produces tritium. Corrosion control and tritium management in the salt are unproven at power.

In this dataset. Kairos Power. See FHR.

Molten-salt reactor (fuel dissolved in salt)

How it works. The uranium, and sometimes thorium or plutonium, is dissolved as a fluoride or chloride in the salt itself. The fuel salt circulates through the core, where it is critical, and out to a heat exchanger. Designs may be thermal, with a graphite moderator, or fast, with none.

Where it has run. The Molten-Salt Reactor Experiment at Oak Ridge National Laboratory operated from 1965 to 1969 at up to 7.4 MW thermal. China’s TMSR-LF1, a 2 MW thermal research reactor in Gansu province, first reached criticality on 11 October 2023 and reached full power on 17 June 2024. Thorium was first loaded into its fuel salt in October 2024, and from 8 October it ran at full power for 10 days with thorium in the salt.

Main advantage. No fuel fabrication, low pressure, and the option to add fuel and remove some fission products while running.

Main open problem. The fission products are not held inside a solid fuel. They circulate through the whole primary circuit, so pumps and heat exchangers become highly radioactive and must be maintained remotely, and every part of the loop must resist corrosion by the salt for decades.

In this dataset. Copenhagen Atomics, Moltex Energy, Natura Resources, Saltfoss Energy, Stellaria, Terrestrial Energy, ThorCon International, Thorizon, Flibe Energy, NAAREA (in liquidation since January 2026), the Shanghai Institute of Applied Physics and Abilene Christian University. Moltex’s designs are an edge case: the fuel salt sits in vented fuel tubes, similar to the fuel pins of a conventional reactor, and a separate salt carries the heat away. See molten-salt.

Sodium-cooled fast reactor (SFR)

How it works. Liquid sodium carries the heat at near-atmospheric pressure, with no moderator, so the neutrons stay fast. Fuel is usually a metal alloy or oxide with more fissile material than light-water fuel. TerraPower’s Natrium design specifies metallic uranium alloy of up to 19.75% enrichment, and a 345 MWe rating.

Where it runs or has run. EBR-II in Idaho ran from 1964 to 1994. Russia operates the two fast reactors counted in the fleet table above. TerraPower received a US construction permit for Kemmerer Unit 1, a Natrium plant in Wyoming, on 9 March 2026.

Main advantage. Sodium carries heat very well without pressurisation, and a fast spectrum can breed new fuel or fission long-lived heavy elements.

Main open problem. Sodium burns in air and reacts violently with water, so leaks must be prevented and contained. Large sodium cores can also have a positive void coefficient that the core design must offset.

In this dataset. TerraPower, Oklo, GE Vernova Hitachi, ARC Clean Technology, Toshiba, Zap Energy (a fusion company that also develops a sodium-cooled fission reactor based on Toshiba’s 4S), HEXANA, Otrera, India’s Indira Gandhi Centre for Atomic Research and the China Institute of Atomic Energy. Aalo Atomics is also sodium-cooled, but its Aalo-1 design is described by the company as “Sodium Thermal” with uranium oxide fuel, so its neutrons are slowed and it is not a fast reactor. It is classed only as liquid-metal. See SFR.

Lead-cooled fast reactor (LFR)

How it works. Molten lead, or a lead-bismuth mix, carries the heat, with a fast spectrum and no pressurisation. newcleo’s LFR-AS-200 design specifies lead at 420 °C to 530 °C and mixed uranium-plutonium oxide (MOX) fuel.

Where it runs or has run. Soviet Alfa-class submarines, in service from 1971 to 1996, used lead-bismuth-cooled reactors, although these were moderated with beryllium and so were not fast reactors. Russia began building BREST-OD-300, a 300 MWe lead-cooled demonstration plant, on 8 June 2021.

Main advantage. Lead does not burn in air or react violently with water, unlike sodium.

Main open problem. Corrosion of steel by liquid lead, the weight of the coolant, and the need to keep lead above its melting point of 327.5 °C at all times. Lead-bismuth melts lower but produces polonium-210, a strong alpha emitter, under neutron irradiation.

In this dataset. newcleo, Blykalla, Rosatom and the Russian design institute NIKIET. See LFR.

Heat-pipe microreactor

How it works. A solid core, often a block of graphite with fuel compacts, is threaded with sealed heat pipes. Each pipe holds a small amount of liquid metal, usually sodium, that evaporates at the hot end, condenses at the cold end and returns by capillary action. There are no pumps and no coolant loop. Westinghouse’s eVinci design is rated at 5 MWe from a 15 MW thermal core, using TRISO fuel at 19.75% enrichment.

Where it has run. No heat-pipe power reactor has operated. NASA and the US Department of Energy tested KRUSTY, a kilowatt-scale heat-pipe space reactor, at the Nevada National Security Site between November 2017 and March 2018.

Main advantage. Very few moving parts, which suits a small reactor that is shipped whole and runs for years unattended.

Main open problem. Scale and reliability. Each heat pipe carries a limited amount of heat, so the approach is confined to small outputs, and heat pipes have not yet been shown to perform for years inside a power reactor.

In this dataset. Antares Nuclear, NANO Nuclear Energy, Westinghouse and Toshiba. See heat-pipe and microreactors.

Further reading on this site

• Fission segment and all fission technologies
• Small modular reactors and microreactors
• Licensing milestones

Sources

• World Nuclear Association, Nuclear Power Reactors (operable reactor table, updated 5 August 2026; PHWR description)
• IAEA, Small Modular Reactor Technology Catalogue 2024 (Rolls-Royce SMR, BWRX-300, Xe-100, KP-FHR, Natrium, LFR-AS-200, eVinci, HTR-PM, HTTR, HTR-10, BREST-OD-300 and Moltex parameters and status; Hermes milestones)
• World Nuclear Association, Thorium (Peach Bottom, Fort St Vrain, AVR and THTR operating years)
• US Department of Energy, TRISO particles: the most robust nuclear fuel on Earth
• Wikipedia, Void coefficient
• US Department of Energy, What is high-assay low-enriched uranium (HALEU)?
• Wikipedia, FLiBe (lithium-7 requirement, volumetric heat capacity)
• Wikipedia, Molten-Salt Reactor Experiment
• Wikipedia, TMSR-LF1 (quoting the Generation IV International Forum on criticality, full power and thorium operation)
• Wikipedia, Experimental Breeder Reactor II
• Wikipedia, Lead-cooled fast reactor (BREST-OD-300, melting points, polonium-210, Alfa-class beryllium moderator)
• Wikipedia, Alfa-class submarine (years in service)
• Wikipedia, Kilopower (KRUSTY test)
• Westinghouse, eVinci microreactor
• Aalo Atomics’ description of Aalo-1 (“30 MWt / 10 MWe”, “Sodium Thermal”, “UO₂ Ceramic”) is quoted from its website as captured on its company page.
• Regulatory dates for Darlington, Clinch River, Hermes, Hermes 2 and Kemmerer are from this dataset’s licensing milestones, each anchored to the regulator’s own record.