Chalk River NRX
The first reactor accident anywhere to damage a core. A valve opened in error, control rods failed to insert, and a power excursion melted part of the fuel and ruptured the calandria.
| Date | 1952-12-12 |
|---|---|
| INES rating | 5 Accident with wider consequences
Retrospective. INES was introduced in 1990, so this rating is an attribution made
afterwards rather than an assessment a regulator issued at the time.
|
| Facility | research reactor |
| Country | Canada |
| Deaths at the time | 0 |
|---|---|
| Acute radiation deaths | not established |
No deaths. Several hundred military and civilian personnel took part in the cleanup, among them a young United States Navy officer named Jimmy Carter. Doses were recorded but no health effects have been attributed to them.
Operator error compounded by shut-off rods that failed to fall
What happened
An operator opened air valves in the basement that lifted several shut-off rods. A supervisor's instruction to a colleague upstairs was miscommunicated, and the wrong buttons were pressed. Power rose to roughly double the design level in seconds. The core was destroyed and about 4,500 cubic metres of contaminated water flooded the basement.
Consequences
No off-site release of consequence and no injuries. The reactor's calandria was buried and NRX was rebuilt and back in service within two years, which would be unimaginable now.
What changed afterwards
It happened seven years before any commercial reactor operated anywhere, and it established two things the industry took decades to fully absorb: that shutdown systems must be independent and physically reliable, and that a control room and a basement need to be talking about the same valve.
Other accidents
| Accident | INES | Date |
|---|---|---|
| Chernobyl | 7 | 1986-04-26 |
| Fukushima Daiichi | 7 | 2011-03-11 |
| Kyshtym (Mayak) | 6 | 1957-09-29 |
| Windscale Pile 1 fire | 5 | 1957-10-10 |
| Three Mile Island Unit 2 | 5 | 1979-03-28 |
| SL-1 | 4 | 1961-01-03 |
| Lucens | 4 | 1969-01-21 |
| Saint-Laurent A1 fuel melt | 4 | 1969-10-17 |
| Bohunice A1 refuelling accident | 4 | 1977-02-22 |
| Saint-Laurent A2 fuel melt | 4 | 1980-03-13 |
| Tokaimura criticality accident | 4 | 1999-09-30 |
| Browns Ferry cable fire | 3 | 1975-03-22 |
| Greifswald cable fire | 3 | 1975-12-07 |
| Vandellòs I turbine fire | 3 | 1989-10-19 |
| Tomsk-7 reprocessing explosion | 3 | 1993-04-06 |
| Davis-Besse vessel head corrosion | 3 | 2002-03-06 |
| Paks fuel cleaning accident | 3 | 2003-04-10 |
| Fukushima Daini | 3 | 2011-03-11 |
Sources
- International Atomic Energy Agency INIS, The NRX accident · retrieved 2026-09-07
How these sources are used, including the corrections applied on top of them and what is deliberately left blank · download the underlying data