RBMK vs PWR: why the Chernobyl design is different
The RBMK is a Soviet pressure-tube reactor: light water boils inside the channels at about 290 °C and the moderator is graphite, not water. That combination gives a positive void coefficient, meaning that more steam in the channels raises reactivity, the underlying problem of the Chernobyl accident. A PWR uses the same water as moderator and coolant, so losing water slows the reaction, and it encloses the primary circuit in a full containment.
Figures from the atlas
| RBMK | PWR | |
|---|---|---|
| Units built or under way | 17 | 481 |
| Operating today | 7 | 318 |
| Net capacity operating | 6,475 MWe | 304,963 MWe |
| Countries | Lithuania, Russia, Ukraine | 38 |
| First grid connection | 1973 | 1957 |
| Most recent connection | 1990 | 2024 |
| Under construction | 0 | 73 |
| Shut down | 10 | 72 |
| Most powerful unit operating | Kursk 1-3 (Russia), 925 MWe | Taishan 1-1 (China), 1,660 MWe |
| Average load factor, latest reported year | 83.7 % | 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
- Moderator: graphite in the RBMK; light water in the PWR.
- Coolant: water boiling in vertical pressure tubes in the RBMK; water at about 150 atmospheres that never boils, in a vessel, in the PWR.
- Void coefficient: positive in the original RBMK design and reduced, not removed, after 1986 with 2.4% U-235 enrichment and up to 90 fixed absorbers in the core; negative by construction in a PWR.
- Containment: the RBMK has no full containment of the kind a Western PWR has.
- Fleet: the remaining RBMKs operate only in Russia; the last one connected to the grid was Smolensk 3, in 1990.
Frequently asked questions
Are any RBMKs still operating?
Yes, all in Russia: at Leningrad, Kursk and Smolensk. Chernobyl and Ignalina are shut down. The table on this page shows how many still operate per the atlas.
What is a positive void coefficient?
That as more steam forms in an RBMK's channels the chain reaction speeds up instead of slowing, because the moderator is graphite rather than water. In a PWR the water is the moderator: lose it and the reaction stops.
Can the safety of the two be compared?
The RBMKs still operating were modified after 1986 and are overseen by the Russian regulator, but the design keeps fundamental differences from a PWR: graphite moderator and no full containment. The World Nuclear Association describes them in its RBMK appendix.
Animated guides
RBMK (The 17 RBMK units in the atlas) · PWR (The 481 PWR units in the atlas) · Compare any two units · Rankings
Sources
- World Nuclear Association, RBMK Reactors
- IAEA, The Chernobyl Accident: Updating of INSAG-1 (INSAG-7)
- World Nuclear Association, Nuclear Power Reactors
- Wikipedia, RBMK
Other comparisons
PWR vs BWR · PWR vs CANDU · CANDU vs VVER · EPR vs AP1000 · AGR vs PWR