PWR vs CANDU (PHWR): natural uranium against enriched uranium
The CANDU and the PWR share the steam cycle and differ in the core. The CANDU is moderated by heavy water, which absorbs so few neutrons that natural uranium can be used, and instead of a vessel it has hundreds of horizontal pressure tubes refuelled without shutting the reactor down. The PWR is moderated by ordinary water, needs uranium enriched to 3.5 to 5% and is refuelled with the plant shut down.
Figures from the atlas
| PWR | CANDU | |
|---|---|---|
| Units built or under way | 479 | 72 |
| Operating today | 316 | 45 |
| Net capacity operating | 302,880 MWe | 23,594 MWe |
| Countries | 38 | 10 |
| First grid connection | 1957 | 1962 |
| Most recent connection | 2026 | 2025 |
| Under construction | 68 | 3 |
| Shut down | 75 | 13 |
| Most powerful unit operating | Taishan 1-1 (China), 1,660 MWe | Darlington 1 (Canada), 878 MWe |
| Average load factor, latest reported year | 82 % | 81.1 % |
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
- Fuel: the CANDU burns natural uranium oxide, 0.7% U-235; the PWR uses uranium enriched to 3.5 to 5%.
- Moderator: heavy water (D₂O) in the CANDU; light water in the PWR.
- Core: several hundred horizontal pressure tubes in the CANDU; a single pressure vessel in the PWR.
- Refuelling: the CANDU is refuelled on power, tube by tube; the PWR shuts down every 12 to 24 months to replace part of the core.
- Burnup: CANDU fuel is around 7.5 GWd/t and does not stay long in the core; PWR fuel now reaches more than 60 GWd/t.
Frequently asked questions
Why can a CANDU use natural uranium?
Because heavy water barely absorbs neutrons, so the chain reaction is sustained with the 0.7% U-235 of natural uranium. With ordinary water too many neutrons are lost and enrichment is needed.
Are the Atucha reactors CANDUs?
No. Atucha 1 and 2 are heavy-water PHWRs but with a pressure vessel, a Siemens-KWU design; Embalse is a CANDU 6. That is why Atucha has its own guide in the wiki.
Which reactor refuels without shutting down?
The CANDU, thanks to its pressure tubes: fuelling machines at both ends swap fuel bundles with the reactor at power. A PWR needs a refuelling outage.
Animated guides
PWR (The 479 PWR units in the atlas) · CANDU (The 72 CANDU units in the atlas) · Compare any two units · Rankings
Sources
- World Nuclear Association, Nuclear Power Reactors
- World Nuclear Association, Nuclear Fuel Cycle Overview
- World Nuclear Association, Nuclear Fuel Fabrication
- Canadian Nuclear Safety Commission, Nuclear power plants
- Wikipedia, CANDU reactor
Other comparisons
PWR vs BWR
Pressurized water against boiling water: pressure, loops, control rods and turbine, with the real fleet of each type computed from the atlas.
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.