AGR vs PWR: gas and graphite against pressurized water
The AGR is the second generation of British gas reactors: graphite as moderator, carbon dioxide as coolant and uranium enriched to 2.5 to 3.5% in stainless steel cladding. It exits at 650 °C, far hotter than a PWR, which gives it a thermal efficiency of about 41%. The PWR runs on water at about 150 atmospheres, exits much cooler and sits around one third efficiency, but it is more compact and was built all over the world.
Figures from the atlas
| AGR | PWR | |
|---|---|---|
| Units built or under way | 15 | 481 |
| Operating today | 8 | 318 |
| Net capacity operating | 4,685 MWe | 304,963 MWe |
| Countries | United Kingdom | 38 |
| First grid connection | 1963 | 1957 |
| Most recent connection | 1989 | 2024 |
| Under construction | 0 | 73 |
| Shut down | 7 | 72 |
| Most powerful unit operating | Heysham B1 (United Kingdom), 620 MWe | Taishan 1-1 (China), 1,660 MWe |
| Average load factor, latest reported year | 68.8 % | 81.9 % |
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
- Coolant and moderator: CO₂ and graphite in the AGR; light water for both jobs in the PWR.
- Temperature and efficiency: 650 °C outlet and about 41% thermal efficiency in the AGR, against roughly one third in a PWR.
- Fuel: uranium enriched to 2.5 to 3.5% in stainless steel cladding in the AGR; 3.5 to 5% in zircaloy cladding in the PWR.
- Refuelling: the AGR was designed for on-load refuelling; the PWR refuels during an outage.
- Spread: the AGR was only ever built in the United Kingdom; the PWR is the most widespread design in the world.
Frequently asked questions
Why is the AGR more efficient?
Because the CO₂ leaves the core at 650 °C and allows steam conditions similar to a coal plant's. A PWR runs its water at about 320 to 330 °C, below its boiling point at about 150 atmospheres, which caps the cycle's efficiency.
Are AGRs still being built?
No. The 14 British AGRs were connected between 1976 and 1989 and several have already shut down; the UK chose the PWR for Sizewell B and the EPR for Hinkley Point C and Sizewell C. The table shows how many still operate.
What gas reactors came before the AGR?
The Magnox reactors, with natural uranium metal fuel and magnesium cladding, which started at Calder Hall in 1956 and are all shut down. The AGR was their successor with enriched uranium and higher temperatures.
Animated guides
AGR (The 15 AGR units in the atlas) · PWR (The 481 PWR units in the atlas) · Compare any two units · Rankings
Sources
- World Nuclear Association, Nuclear Power Reactors
- World Nuclear Association, Nuclear Power in the United Kingdom
- Wikipedia, Advanced gas-cooled reactor
Other comparisons
PWR vs BWR · PWR vs CANDU · CANDU vs VVER · EPR vs AP1000 · RBMK vs PWR