What is the main difference between a PWR and a BWR?

In a PWR the primary water stays liquid under high pressure and makes steam in separate steam generators; in a BWR the water boils in the core and that steam drives the turbine directly.

Why do most reactors use water?

Water is cheap, well understood, and works as both moderator and coolant. Its main drawback is that it must be highly pressurized to stay liquid at useful temperatures.

What is a fast reactor?

A reactor that sustains the chain reaction with fast (unmoderated) neutrons. It needs more fissile material but can breed fuel and fission long-lived actinides.

What is the pressurizer for?

In a PWR it keeps primary system pressure at the set value using electric heaters and water sprays, and it carries relief and safety valves.

Reactor types explained: PWR, BWR, gas-cooled, sodium and molten salt

Reactors are classified mostly by their coolant and moderator. Those two choices drive the pressure, temperature, fuel and safety systems of the whole plant.

Study text drafted from public-domain US government sources. Practice questions are reviewed before publication.

Light-water reactors: the US fleet

Every operating US commercial power reactor is a light-water reactor. In a pressurized-water reactor (PWR), about two-thirds of the fleet, the primary coolant is held at roughly 2,250 psi so it does not boil; it heats separate secondary water in steam generators, and a pressurizer holds the pressure steady. In a boiling-water reactor (BWR), the water boils in the core and the steam goes directly to the turbine, removing the steam generators but bringing slightly radioactive steam into the turbine building.

Both use uranium dioxide pellets enriched to about 3 to 5 percent U-235, sealed in zirconium-alloy cladding. Their thermal efficiency is about one-third, limited by the modest steam temperature. Defense in depth relies on three physical barriers: the fuel cladding, the reactor coolant pressure boundary and the containment.

Other coolants and moderators

Heavy-water reactors use deuterium oxide, which absorbs so few neutrons that natural uranium can be used as fuel. High-temperature gas-cooled reactors use helium coolant and a graphite moderator with TRISO particle fuel, reaching outlet temperatures high enough for industrial heat. Sodium-cooled fast reactors have no moderator and run near atmospheric pressure because sodium boils at about 883 °C. Molten salt reactors use a salt that is liquid at operating temperature as the coolant, and in some designs the fuel is dissolved in the salt.

Many small modular reactor designs are scaled-down, simplified PWRs, while others use these alternative coolants to gain higher temperatures, lower pressures or different fuel cycles. The trade-offs involve materials, chemistry, fuel supply and how much operating experience exists.

Sample quiz

Practice questions for this section are being reviewed.

Practice questions for this section are still in review and will appear here once they have been checked.

Frequently asked questions

What is the main difference between a PWR and a BWR?: In a PWR the primary water stays liquid under high pressure and makes steam in separate steam generators; in a BWR the water boils in the core and that steam drives the turbine directly.
Why do most reactors use water?: Water is cheap, well understood, and works as both moderator and coolant. Its main drawback is that it must be highly pressurized to stay liquid at useful temperatures.
What is a fast reactor?: A reactor that sustains the chain reaction with fast (unmoderated) neutrons. It needs more fissile material but can breed fuel and fission long-lived actinides.
What is the pressurizer for?: In a PWR it keeps primary system pressure at the set value using electric heaters and water sprays, and it carries relief and safety valves.

Sources

U.S. Energy Information Administration, Nuclear explained www.eia.gov
NUREG-1350, NRC Information Digest
U.S. NRC Glossary (NRC Basic References) www.nrc.gov
DOE Fundamentals Handbook: Nuclear Physics and Reactor Theory, DOE-HDBK-1019/1-93 and DOE-HDBK-1019/2-93
U.S. DOE Office of Nuclear Energy, TRISO Particles: The Most Robust Nuclear Fuel on Earth

Explanations on this page are original. Cited US government works are in the public domain; they do not endorse this site.