| Level, Descriptor | Off-Site Impact | On-Site Impact | Defence-in-Depth Degradation | Examples |
| 7 Major Accident |
Major Release: Widespread health and environmental effects | Chernobyl, Ukraine, 1986 (fuel meltdown and fire) | ||
| 6 Serious Accident |
Significant Release: Full implementation of local emergency plans | Mayak at Ozersk, Russia, 1957 (reprocessing plant criticality) | ||
| 5 Accident with Off-Site Risks |
Limited Release: Partial implementation of local emergency plans | Severe damage to reactor core or to radiological barriers | Windscale, UK, 1957 (military). Three Mile Island, USA, 1979 (fuel melting). | |
| 4 Accident Mainly in Installation either of: |
Minor Release: Public exposure of the order of prescribed limits | Significant damage to reactor core or to radiological barriers, worker fatality |
Saint-Laurent, France, 1980
(fuel rupture in reactor). Tokai-mura, Japan, 1999 (criticality in fuel plant for an experimental reactor). | |
| 3 Serious Incident any of: |
Very Small Release: Public exposure at a fraction of prescribed limits | Major contamination, Acute health effects to a worker | Near Accident. No safety layers remaining |
Vandellos, Spain, 1989
(turbine fire, no radioactive contamination). Davis-Besse, USA, 2002 (severe corosion) Paks, Hungary, 2003 (fuel damage) |
| 2 Incident |
nil | Significant spread of contamination, Overexposure of worker | Incidents with significant failures in safety provisions | |
| 1 Anomaly |
nil | nil | Anomaly beyond the authorised operating regime | |
| 0 | nil | nil | No safety significance | |
| Below Scale | nil | nil | No safety relevance |
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