Level | Description | Criteria | Example |
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Accidents |
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7 | Major accident | External release of a large fraction of the radioactive material in a large facility (eg, the core of a power reactor). This would typically involve a mixture of short- and long-lived radioactive fission products (in quantities radiologically equivalent to more than tens of thousands of terabecquerels of iodine-131). Such a release would result in the possibility of acute health effects, delayed health effects over a wide area, possibly involving more than one country; and long-term environmental consequences. | Chernobyl nuclear power plant, former USSR (now in Ukraine), 1986 |
6 | Serious accident | External release of radioactive material (in quantities radiologically equivalent to the order of thousands to tens of thousands of terabecquerels of iodine-131). Such a release would be likely to result in full implementation of countermeasures covered by local emergency plans to limit serious health effects. | Kyshtym reprocessing plant, former USSR (now in Russian Federation) 1957 |
5 | Accident with off-site risk | External release of radioactive material (in quantities radiologically equivalent to the order of hundreds to thousands of terabecquerels of iodine-131). Such a release would be likely to result in partial implementation of countermeasures covered by emergency plans to lessen the likelihood of health effects. | Windscale pile, UK, 1957 |
Severe damage to the nuclear facility. This may involve severe damage to a large fraction of the core of a power reactor, a major criticality accident or a major fire or explosion releasing large quantities of radioactivity within the installation. | Three Mile Island, USA, 1979 |
4 | Accident without significant off-site risk | External release of radioactivity resulting in a dose to the most exposed individual off-site of the order of a few millisieverts.* With such a release the need for off-site protective actions would be generally unlikely except possibly for local food control. |   |
Significant damage of the nuclear facility. Such an accident might include damage to nuclear plant leading to major on-site recovery problems such as partial core melt in a power reactor and comparable events at non-reactor installations. | Windscale Reprocessing Plant, UK, 1973; Saint-Laurent nuclear power plant, France, 1980; Buenos Aires Critical Assembly, Argentina, 1983 |
Irradiation of one or more workers which results in an overexposure where a high probability of early death occurs. |   |
Incidents |
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3 | Serious incident | External release of radioactivity above authorised limits, resulting in a dose to the most exposed individual off-site of the order of tenths of millisieverts.* With such a release, off-site protective measures may not be needed. |   |
On-site events resulting in doses to workers sufficient to cause acute health effects and/or an event resulting in a severe spread of contamination, for example a few thousand terabecquerels of activity released in a secondary containment where the material can be returned to a satisfactory storage area. |
Incidents in which a further failure of safety systems could lead to accident conditions, or a situation in which safety systems would be unable to prevent an accident if certain initiators were to occur. | Vandellos nuclear power plant, Spain, 1989 |
2 | Incident | Incident with significant failure in safety provisions but with sufficient defence in depth remaining to cope with additional failures. |   |
An event resulting in a dose to a worker exceeding a statutory annual dose limit and/or an event which leads to the presence of significant quantities of radioactivity in the installation in areas not expected by design and which require corrective action. |
1 | Anomaly | Anomaly beyond the authorised operating regime. This may be due to equipment failure, human error or procedural inadequacies. (Such anomalies should be distinguished from situations where operational limits and conditions are not exceeded and which are properly managed in accordance with adequate procedures. These are typically 'below scale'.) |   |
Below scale/zero | Deviation | No safety significance |   |