Lucens
Switzerland's only home-built power reactor, in an underground cavern, suffered a partial core melt and was never restarted. It ended the Swiss attempt at an indigenous design.
| Date | 1969-01-21 |
|---|---|
| INES rating | 4 Accident with local 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 | power reactor |
| Unit | Lucens nuclear power plant |
| Reactor type | Pressurised heavy water reactor (PHWR) |
| Country | Switzerland |
| Deaths at the time | 0 |
|---|---|
| Acute radiation deaths | not established |
No deaths and no injuries. The cavern was evacuated and sealed.
Corrosion of a fuel channel leading to coolant blockage
What happened
A magnesium alloy fuel element corroded during a shutdown, blocking coolant flow to one channel. On restart the channel overheated, the fuel melted, and the pressure tube ruptured, releasing carbon dioxide coolant and radioactivity into the cavern.
Consequences
Contamination was contained within the underground cavern, which had been built into rock partly for that reason. Nobody was hurt. The reactor was written off.
What changed afterwards
Switzerland abandoned indigenous reactor development and bought American and German designs instead. The cavern was decontaminated over decades and now houses an archive. It is the strongest argument on record for siting a prototype underground.
Other accidents
| Accident | INES | Date |
|---|---|---|
| Chernobyl | 7 | 1986-04-26 |
| Fukushima Daiichi | 7 | 2011-03-11 |
| Kyshtym (Mayak) | 6 | 1957-09-29 |
| Chalk River NRX | 5 | 1952-12-12 |
| Windscale Pile 1 fire | 5 | 1957-10-10 |
| Three Mile Island Unit 2 | 5 | 1979-03-28 |
| SL-1 | 4 | 1961-01-03 |
| 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, Accident at the experimental nuclear power station in Lucens · 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