SL-1
A small US Army experimental reactor destroyed itself in milliseconds when one control rod was pulled too far, killing all three operators. The only fatal reactor accident in US history.
| Date | 1961-01-03 |
|---|---|
| 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 | research reactor |
| Country | United States |
| Deaths at the time | 3 |
|---|---|
| Acute radiation deaths | 0 |
All three operators on shift were killed instantly, the only fatal reactor accident in United States history.
A single central control rod was withdrawn far beyond its normal position during maintenance, taking the reactor prompt critical.
What happened
SL-1 was a compact reactor prototype for remote military installations. Its design had a fundamental weakness: the central control rod alone had enough reactivity worth to take the reactor critical if fully withdrawn.
During reassembly after maintenance, that rod was withdrawn roughly 50 cm instead of the intended few centimetres. The reactor went prompt critical, the water flashed to steam, and the hydraulic shock lifted the entire vessel several feet off the floor.
Consequences
All three men on shift died. Recovery of the bodies and the site took over a year and the reactor was buried.
What changed afterwards
Every subsequent US reactor design has been required to ensure that no single control rod can produce criticality on withdrawal, the "one stuck rod" criterion. It is a rule written directly from this accident.
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 |
| 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
- US Department of Energy and IAEA INES documentation · 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