Davis-Besse vessel head corrosion
Nothing happened, and that is why it matters. Corrosion had eaten a football-sized hole through a reactor vessel head, leaving a 6 mm stainless liner as the only barrier — discovered by chance.
| Date | 2002-03-06 |
|---|---|
| INES rating | 3 Serious incident |
| Facility | power reactor |
| Unit | Davis Besse nuclear power plant 1 |
| Reactor type | Pressurised water reactor (PWR) |
| Country | United States |
| Deaths at the time | 0 |
|---|---|
| Acute radiation deaths | not established |
No deaths, injuries or release. This is a near-miss, included because of what it revealed.
Boric acid leaking from a cracked nozzle ate a cavity through the carbon steel reactor vessel head over years, leaving only the thin stainless steel liner holding the pressure.
What happened
Boric acid in the coolant leaked from cracked control rod drive nozzles and, over several years, corroded away the carbon steel of the reactor pressure vessel head. Inspectors eventually found a cavity roughly the size of a football.
What remained holding back a reactor coolant system at 155 bar was the vessel's stainless steel cladding, about 6 mm thick, already visibly bulging.
Consequences
The plant was shut for two years. The operator was fined and later pleaded guilty to concealing information from the regulator, having deferred inspections that would have found the damage.
What changed afterwards
Davis-Besse is the industry's reference case for normalisation of deviance: boric acid deposits had been seen and recorded repeatedly and treated as a housekeeping nuisance rather than as evidence of an active leak.
It changed inspection requirements for vessel heads across the US fleet and is taught as a regulatory failure as much as an engineering one — the NRC had accepted the operator's case for delaying an inspection.
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 |
| Saint-Laurent A2 fuel melt | 4 | 1980-03-13 |
| Tokaimura criticality accident | 4 | 1999-09-30 |
| Vandellòs I turbine fire | 3 | 1989-10-19 |
Sources
- US Nuclear Regulatory Commission · retrieved 2026-09-07