PWR vs BWR: how they differ and how many there are
Both light-water reactors use enriched uranium and ordinary water as moderator and coolant. The difference is where the steam is made: a PWR keeps the core water at about 150 atmospheres so it never boils and raises steam in separate generators; a BWR lets it boil inside the core, at about 75 atmospheres, and sends that steam straight to the turbine.
Figures from the atlas
| PWR | BWR | |
|---|---|---|
| Units built or under way | 479 | 120 |
| Operating today | 316 | 60 |
| Net capacity operating | 302,880 MWe | 60,994 MWe |
| Countries | 38 | 13 |
| First grid connection | 1957 | 1957 |
| Most recent connection | 2026 | 2005 |
| Under construction | 68 | 2 |
| Shut down | 75 | 55 |
| Most powerful unit operating | Taishan 1-1 (China), 1,660 MWe | Grand Gulf 1 (United States), 1,401 MWe |
| Average load factor, latest reported year | 82 % | 86 % |
Data: Global Energy Monitor (CC BY 4.0), IAEA PRIS 2025, Wikipedia (CC BY-SA), Wikidata and Wikimedia Commons. Capacities in net MWe unless stated.
Differences
- Pressure: a PWR primary circuit runs at about 150 times atmospheric pressure; a BWR at about 75.
- Loops: a PWR has two to four loops in Western designs, six in the VVER-440, each with a steam generator; a BWR is a direct cycle with no steam generators, and the steam passes through drier plates before the turbine.
- Turbine: in a BWR the steam driving the turbine comes from the core, so the turbine hall is a controlled area; in a PWR the core water never leaves the primary circuit.
- Control rods: they enter from the top in a PWR; in a BWR they are cruciform blades inserted from the bottom, because the steam driers sit on top.
- Fleet: the PWR is the dominant design worldwide, with more than three hundred reactors operating; BWRs are a few dozen, and the table above gives the atlas's exact figures.
Frequently asked questions
Which is more common, PWR or BWR?
The PWR, by far: more than three hundred operating worldwide against a few dozen BWRs. The table on this page shows the atlas's exact figures.
Is a BWR less safe than a PWR?
They are not different safety categories: both are licensed to the same criteria and both have containment. They differ in where steam is raised and in which parts of the plant see core fluid.
Why does a BWR run at lower pressure?
Because it needs the water to boil in the core: at about 75 atmospheres water boils at about 290 °C. A PWR needs exactly the opposite, no boiling, which is why it raises the pressure to about 150 atmospheres.
Animated guides
PWR (The 479 PWR units in the atlas) · BWR (The 120 BWR units in the atlas) · Compare any two units · Rankings
Sources
- World Nuclear Association, Nuclear Power Reactors
- Wikipedia, Pressurized water reactor
- Wikipedia, Boiling water reactor
Other comparisons
PWR vs CANDU
Heavy water and pressure tubes against a pressure vessel and enriched uranium: fuel, moderator, refuelling and the real fleet of each type.
CANDU vs VVER
The Canadian PHWR against the VVER, the Russian PWR: fuel, core geometry, steam generators and the real fleet of each design.
EPR vs AP1000
The EPR and the AP1000, the West's two large Generation III+ PWRs: power, loops, containment, safety systems and where they were built.
AGR vs PWR
Britain's CO₂-cooled reactor against the world standard: temperature, efficiency, fuel, refuelling and the real fleet.
RBMK vs PWR
Graphite and pressure tubes with boiling water against a vessel and pressurized water: void coefficient, containment and how many RBMKs still operate.