Types of reactors

Commercial reactors around the world

There are six main reactor types in use around the world. The various designs use different concentrations of uranium for fuel, different moderators to slow down the fission process, and different coolants to transfer heat.

The most common reactor type is the pressurized water reactor (PWR), representing 290 of the world’s 447 reactors now operating.

Commercial reactor types around the world
Reactor type Fuel Moderator Coolant Number
Pressurized water reactor (PWR) Enriched UO2 Water Water 290
Boiling water reactor (BWR) Enriched UO2 Water Water 78
Pressurized heavy water reactor (PHWR) Natural UO2 Heavy water Heavy water 47
Light water graphite reactor (LWGR) Enriched UO2 Graphite Water 15
Gas-cooled reactor (GCR) Natural U, enriched UO2 Graphite Carbon dioxide 14
Fast breeder reactor (FBR) PuO2 and UO2 None Liquid sodium 3

Source: World Nuclear Association.

Pressurized water reactors


Pressurized water reactor. Source: Cameco.

Pressurized water reactors (PWRs) are the most common type of reactor worldwide. PWRs use ordinary (or “light”) water as both coolant and moderator. The coolant is pressurized to stop it from flashing into steam to keep it liquid during operation. Powerful pumps circulate the water through pipes, transferring heat that boils water in a separate, secondary loop. The resulting steam drives the electricity-producing turbine generators.

The process of generating power with PWRs is demonstrated in a YouTube video.

Boiling water reactors

Boiling water reactors (BWRs) make up 15% of reactors globally. In a BWR, light water acts as both coolant and moderator. The coolant is kept at a lower pressure than in a PWR, allowing it to boil. The steam is passed directly to the turbine generators to produce electricity. While the absence of a steam generator simplifies the design, radioactivity can contaminate the turbine.

The process of generating power with BWRs is demonstrated in a YouTube video.

Pressurized heavy water reactors


Boiling water reactor. Source: Cameco.

Also known as CANDU reactors, pressurized heavy water reactors (PHWRs) represent about 12% of the reactors in the world and are used at all Canadian nuclear power generation stations. They use heavy water as both coolant and moderator, and use natural uranium as fuel. As in a PWR, the coolant is used to boil ordinary water in a separate loop. CANDU reactors can be refuelled without shutting the reaction down.

The CANDU process is demonstrated in a YouTube video.

Gas-cooled reactors

Gas-cooled reactors (GCRs) are in use only in the United Kingdom. There are two types, the Magnox (named from the magnesium alloy used to clad the fuel elements) and the advanced gas-cooled reactor (AGR). Both types use carbon dioxide as the coolant and graphite as the moderator. The Magnox uses natural uranium as fuel, while the AGR uses enriched uranium. Like CANDU reactors, these designs can be refueled while operating.

Light water graphite reactors

Light water graphite reactors (LWGRs) are used in Russia, with ordinary water as the coolant and graphite as the moderator. As with BWRs, the coolant boils as it passes through the reactor and the resulting steam is passed directly to turbine generators. Early LWGR designs were often built and operated without the safety characteristics and features required elsewhere. The well known 1986 accident at Chernobyl (Ukraine) happened to a reactor of this type.

Features of LWGRs are described in a YouTube video.

Fast breeder reactors


Fast breeder reactor. Source: Cameco.

Because slow neutrons are more likely to split uranium atoms, most reactor types are designed to make use of them. In contrast, fast breeder reactors (FBRs) use fast neutrons to convert materials such as uranium-238 and thorium-232 into fissile materials, which then fuel the reactor. This process, combined with recycling, has the potential to increase available nuclear fuel resources in the very long term. FBRs operate mainly in Russia.

Small modular reactors

The modern small modular reactor (SMR) is designed to be built economically in factory-like conditions (rather than onsite), and with capacities between approximately 10 MWe and 300 MWe.

There is growing interest in SMRs to provide electricity to service small electricity grids, and possibly to provide heat for resource industries. SMRs can also be added incrementally to larger grids as demand grows. The IAEA estimates that as many as 96 SMRs could be operational worldwide by 2030.

Some SMR designs are in advanced stages of development, including several designed to be fully underground, minimizing land use, staffing, and security needs. Some designs include passive safety systems, and can operate for up to four years without refuelling.


B&W mPower small modular reactor. Source: Babcock & Wilcox.