Level Descriptor Criteria Examples
7 Major Accident ¤ External release of a large fraction of the reactor core inventory typically Chernobyl involving a mixture of short and long-lived radioactive fission products USSR, 1986 (in quantities radiologically equivalent to more than tens of thousands terabecquerels of iodine-131). Possibility of acute health effects. Delayed health effects over a wide area, possibly involving more than one country. Long-term environmental consequences. 6 Serious Accident ¤ External release of fission products (in quantities radiologically equivalent to the order of thousands to tens of thousands of terabecquerels of iodine-131). Full implementation of local emergency plans most likely needed to limit serious health effects. 5 Accident with ¤ External release of fission products (in quantities radiologically Windscale, Off-Site Risks equivalent to the order of hundreds to thousands of terabecquerels of UK, 1957 iodine-131). Partial implementation of emergency plans (e.g. local sheltering and/or evacuation) required in some cases to lesson the likelihood of health effects. ¤ Severe damage to large fraction of the core due to mechanical effects Three Mile and/or melting. Island, 1979 4 Accident Mainly ¤ External release of radioactivity resulting in a dose to the most exposed in Installation individual off-site on the order of a few millisieverts.¹ Need for off-site protective actions generally unlikely except possibly for local food control. ¤ Some damage to reactor core due to mechanical effects and/or melting. Saint-Laurent, ¤ Worker doses that can lead to acute health effects (on the order of 1 France, 1980 sievert).² 3 Serious Incident ¤ External release of radioactivity above authorized limits, resulting in a dose to the most exposed individual off-site on the order of tenths of a millisieverts.¹ Off-site protective measures not needed. ¤ High radiation levels and/or contamination on-site due to equipment failures or operational incidents. Overexposure of workers (individual dose exceeding 50 millisieverts).² ¤ Incidents in which a further failure of safety systems could lead to Vandellos, accident conditions, or a situation in which safety systems would be Spain, 1989 unable to prevent an accident if certain initiators were to occur. 2 Incident ¤ Technical incidents or anomalies which, although not directly or imme- diately affecting plant safety, are liable to lead to subsequent re-evalua- tion of safety provisions. 1 Anomaly ¤ Functional or operational anomalies which do not pose a risk but which indicate a lack of safety provisions. 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".) 0 No Safety Below Significance scale ¹ The doses are expressed in terms of effective dose equivalent (whole body dose). Those criteria where appropriate also can be expressed in terms of corresponding annual effluent discharge limits authorized by national authorities. ² These doses are also expressed, for simplicity, in terms of effective dose equvalents (sieverts), although the doses in the range involving acute health effects should be expressed in terms of absorbed dose(grays). Source: International Atomic Energy Agency
For the underlying logic of the scale, click here.