Reactor type 2 of 10
Boiling water reactor (BWR)
by Joe Miller
The boiling water reactor lets the water boil right in the core and sends that steam straight to the turbine. One loop instead of two means no steam generators and no pressurizer.
Plain-English explainer by Joe Miller, a nuclear engineer. Not investment advice. Figures are the latest public numbers we could find and change fast; sources are at the bottom of each page.
What it looks like
How the coolant circulates
- Feedwater enters the vessel and mixes with recirculating water.
- Jet pumps drive the water down the outside of the core and up through the fuel.
- About 10 to 15% of the water boils as it passes through the core.
- Separators and dryers remove water droplets; dry steam (about 550°F, 1,000 psi) goes to the turbine.
- Steam is condensed and pumped straight back into the vessel as feedwater.
The main components
| Component | What it does |
|---|---|
| Reactor vessel | Taller than a PWR vessel (about 70 feet), holding the core in the bottom and steam separators and dryers on top. |
| Core | About 600 to 800 fuel bundles, each in a square zirconium channel, with uranium oxide enriched to about 4 to 5%. |
| Control rods | Cross-shaped blades that enter from the bottom, driven by hydraulic control rod drives. (Joe Miller's patent covers a locking-piston control rod drive valve.) |
| Steam separators and dryers | Spin and dry the steam so only dry steam goes to the turbine. |
| Recirculation pumps and jet pumps | Push water down around the core and back up through it. Changing recirculation flow is how operators raise and lower power. |
| Turbine, generator and condenser | The turbine runs on steam that came through the core, so it is shielded and its building is a controlled area. |
| Containment and suppression pool | A large pool of water inside containment condenses steam in an accident, keeping pressure down. River Bend uses the Mark III design. |
| Emergency core cooling | High-pressure core spray, low-pressure core spray, low-pressure injection and an automatic depressurization system. |
Key details
| Item | Detail |
|---|---|
| Coolant and moderator | Light water, boiling in the core |
| Fuel | Uranium oxide, about 4 to 5% enriched |
| Pressure | About 1,000 psi |
| Temperature | About 550°F steam |
| Efficiency | About 33% |
| Typical size | 500 to 1,400 MWe |
| Refueling | Every 18 to 24 months |
How it is maintained
- Refueling outages every 18 to 24 months: the dryer and separator are lifted out underwater and about a third of the fuel is replaced.
- Control rod drive overhauls and scram timing tests.
- Turbine maintenance under radiation controls, because the turbine sees slightly radioactive steam (mostly short-lived nitrogen-16).
- Vessel internals inspections (jet pumps, core shroud, steam dryer) with underwater cameras.
- Suppression pool cleaning and strainer inspections.
Where it runs
About a third of U.S. reactors (31), including River Bend, Grand Gulf, Browns Ferry, Limerick and Clinton, plus plants in Japan, Sweden, Finland, Spain, Switzerland, Mexico and Taiwan. The BWRX-300 small reactor is being built at Darlington, Ontario.
Who's writing this? I'm Joe Miller, a nuclear engineer since 1974 and an ASME Fellow. I've run safety analyses for PWRs and BWRs, directed nuclear engineering at a working BWR, and reviewed new reactor designs for the NRC. This site explains every kind of reactor in plain English. My background and papers.