Reactor types side by side

Every reactor makes the same choice three times: what fuel, what carries the heat away (the coolant), and what slows the neutrons (the moderator, or nothing). Those three choices decide almost everything else, from operating pressure to what kind of waste comes out. When a company announces a "new" reactor, sort it by those three answers first and the marketing gets much easier to read.

The big comparison

Counts below are operating reactors worldwide from the World Nuclear Association's reactor table, checked October 2026. Advanced designs that are licensed or under construction but not yet running show zero or one.

Type	Fuel	Coolant	Moderator	Operating worldwide	Where you see it
Pressurized water (PWR)	Enriched uranium oxide, 3 to 5% U-235	Water under about 150 atmospheres	Water	About 316	Most U.S. plants, France, China, Vogtle 3 and 4, NuScale's design
Boiling water (BWR)	Enriched uranium oxide	Water, boiling in the core	Water	About 60	About a third of U.S. reactors, Fukushima Daiichi, the GE Vernova Hitachi BWRX-300
Pressurized heavy water (CANDU and similar)	Natural uranium oxide	Heavy water	Heavy water	About 47	Canada, India, Argentina, Romania
Gas-cooled (AGR and HTGR)	Uranium oxide or TRISO particles	Carbon dioxide or helium	Graphite	About 9	Britain's AGRs, China's HTR-PM, X-energy's Xe-100 design
Light water graphite (RBMK)	Enriched uranium oxide	Water	Graphite	About 7	Russia only; the Chernobyl type
Fast neutron	Uranium or uranium-plutonium, often metal or mixed oxide	Liquid sodium (or lead)	None	About 2	Russia's BN reactors; TerraPower Natrium, Oklo Aurora designs; Idaho's EBR-I and EBR-II
Molten salt	TRISO pebbles in salt, or uranium dissolved in salt	Molten fluoride or chloride salt	Graphite, or none	0 commercial	Kairos Hermes (salt-cooled, under construction); Oak Ridge's 1960s experiment

About 441 power reactors were running worldwide by the WNA table's count. PWRs and BWRs together, the light water reactors, are about 85 percent of them.

Pressurized water reactors

In a PWR the water that touches the fuel is held at roughly 150 atmospheres, so it reaches about 315 °C (600 °F) without boiling. That hot water flows through thousands of tubes inside a steam generator, where it heats a second, separate loop of water that does boil. The steam spins the turbine. The radioactive primary water never leaves the containment building.

PWRs dominate because the U.S. Navy chose the design for submarines in the 1950s, and the commercial industry grew out of that program. Strengths: decades of operating experience, a huge supply chain, and the negative feedback of water moderation. Weaknesses: the high pressure demands thick steel vessels and piping, and losing coolant is the central accident to design against. Vogtle 3 and 4 in Georgia are Westinghouse AP1000s, a PWR with passive safety systems that use gravity and natural circulation for cooling during an emergency.

Boiling water reactors

A BWR runs at about half the pressure of a PWR and lets the water boil right in the core. The steam goes to the turbine directly. That removes the steam generator, which is simpler, but it means the turbine building sees slightly radioactive steam and needs shielding. Fukushima Daiichi's damaged units were early General Electric BWRs.

The newest BWR design in North America is the GE Vernova Hitachi BWRX-300, about 300 MWe, which circulates its water by natural convection instead of large pumps. Ontario Power Generation is building the first ones at Darlington in Canada, and in late September 2026 the NRC issued the Tennessee Valley Authority a construction permit for one at Clinch River near Oak Ridge. Lesson 4 picks up from there.

CANDU and heavy water

Canada went its own way. Heavy water absorbs so few neutrons that CANDU reactors can run on natural uranium, skipping enrichment entirely. They also refuel while running, pushing fresh fuel bundles in one end of horizontal tubes while spent ones come out the other. The trade-offs: heavy water is expensive, and the design produces more tritium, a radioactive form of hydrogen that has to be managed.

Gas-cooled reactors

Gas does not boil, so gas-cooled reactors can run much hotter than water reactors without high pressure steam. Britain's advanced gas-cooled reactors use carbon dioxide and graphite. The newer high-temperature gas-cooled reactor (HTGR) uses helium and TRISO fuel: poppy-seed sized uranium kernels coated in carbon and silicon carbide layers, packed into pebbles or blocks. TRISO particles are designed to hold their fission products at temperatures above 1,600 °C, which is the basis for claims that these reactors cannot melt in the usual sense.

X-energy's Xe-100 is an HTGR rated at 80 MWe per module. Dow's Long Mott project in Texas would use four of them for both electricity and industrial steam. As of a May 2026 Power Magazine report, the NRC had finished the environmental review and planned to complete its safety evaluation by November 2026.

Fast reactors

Take away the moderator and the neutrons stay fast. Fast neutrons are worse at splitting U-235, so the fuel must be richer, but they can do two things slow neutrons cannot do well. They convert U-238 into plutonium-239, which is fissile, so a fast reactor can "breed" new fuel. And they can split heavier, long-lived elements in spent fuel, shortening how long some waste stays dangerous.

Water would slow the neutrons, so fast reactors use liquid metal. Sodium is the usual choice: it carries heat superbly and stays liquid at normal pressure far above operating temperature. It also burns in air and reacts violently with water, which is the main engineering headache. Idaho has deep history here. EBR-I near Arco was a fast breeder reactor, and on December 20, 1951, it lit four light bulbs, the first usable electricity from a reactor, according to INL.

Two of the best-known advanced projects in the United States are fast reactors. TerraPower's Natrium in Kemmerer, Wyoming, is a 345 MWe sodium-cooled fast reactor whose heat goes into a tank of molten salt, so the plant can surge to 500 MWe for a few hours when the grid needs it. It received its NRC construction permit in March 2026. Oklo's Aurora is a smaller sodium-cooled fast reactor, now 75 MWe, being built at Idaho National Laboratory under DOE authority.

Molten salt reactors

"Molten salt reactor" covers two quite different ideas, and coverage blurs them constantly.

• Salt-cooled, solid fuel: the fuel stays in solid TRISO pebbles and molten fluoride salt carries the heat away. The salt works at near atmospheric pressure and boils at well over 1,000 °C, so there is no high-pressure steam inside the reactor. Kairos Power's Hermes reactors in Oak Ridge are this type.
• Fuel dissolved in salt: the uranium (or thorium) is mixed into the salt itself and circulates. Oak Ridge National Laboratory ran the Molten Salt Reactor Experiment from 1965 to 1969. Several startups are working on this approach, but none was operating commercially as of October 2026.

Salt is corrosive at high temperatures, and qualifying materials that last decades in it is one of the main hurdles for both versions.