Reactor type 1 of 10
Pressurized water reactor (PWR)
by Joe Miller
The pressurized water reactor is the most common power reactor in the world. Its defining feature is two separate water loops: the water that touches the fuel is kept under such high pressure that it never boils, and it heats a second loop of water that makes the steam.
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
- Fuel heats the primary water as it flows up through the core.
- Hot water (about 600°F) leaves through the hot leg to the steam generators.
- Inside the steam generator tubes it gives up its heat, boiling the separate secondary water outside the tubes.
- The cooled primary water (about 550°F) returns through the reactor coolant pumps and cold leg to the vessel.
- Secondary steam drives the turbine, is condensed, and feedwater pumps send it back to the steam generators.
The main components
| Component | What it does |
|---|---|
| Reactor vessel | A steel vessel about 8 inches thick, 40 feet tall, holding the core and the high-pressure water. |
| Core and fuel assemblies | About 150 to 200 assemblies, each a square bundle of roughly 264 zirconium-alloy rods filled with uranium oxide pellets enriched to about 4 to 5%. |
| Control rods | Rods of silver-indium-cadmium or boron carbide that drop in from the top to shut the reactor down; day-to-day power is also trimmed with boric acid dissolved in the water. |
| Pressurizer | A tall tank with electric heaters and sprays that holds the system at about 2,250 psi so the water stays liquid at about 600°F. |
| Steam generators (2 to 4) | Huge heat exchangers with thousands of U-shaped tubes. Hot reactor water flows through the tubes and boils the clean secondary water around them. |
| Reactor coolant pumps | Pumps of 6,000 to 9,000 horsepower each that push primary water around the loop at about 100,000 gallons per minute per pump. |
| Turbine, generator and condenser | Steam spins the turbine and generator, then is condensed back to water by cooling water from a river, lake or cooling tower. |
| Containment | A steel-lined, reinforced-concrete building, typically about 4 feet thick, around the whole primary system. |
| Emergency core cooling | High- and low-pressure injection pumps and accumulator tanks that refill the core if a pipe breaks. Newer AP1000s use gravity-driven passive tanks instead of pumps. |
Key details
| Item | Detail |
|---|---|
| Coolant and moderator | Ordinary (light) water |
| Fuel | Uranium oxide, about 4 to 5% enriched |
| Pressure | About 2,250 psi primary; about 1,000 psi secondary |
| Temperatures | About 550°F in, 600°F out |
| Efficiency | About 33 to 34% |
| Typical size | 900 to 1,400 MWe |
| Refueling | Every 18 to 24 months, about a third of the core |
How it is maintained
- Refueling outages every 18 to 24 months, about 3 to 5 weeks: the vessel head comes off, a third of the fuel is replaced, and the core is reshuffled.
- Steam generator tube inspections with eddy-current probes run through every tube; leaking or thinning tubes are plugged. Many older plants have replaced whole steam generators.
- Reactor coolant pump seal and motor overhauls on a rotating schedule.
- Reactor vessel and weld inspections by remote ultrasonic tools on 10-year intervals, plus surveillance samples that track how neutrons embrittle the steel.
- Water chemistry control (boron, lithium, hydrogen) every shift to limit corrosion.
Where it runs
About 7 in 10 of the world's reactors, and about 63 of the 94 in the U.S., including Vogtle, Palo Verde, Diablo Canyon and South Texas. Westinghouse, Framatome, KEPCO and China's Hualong One are PWR designs.
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.