The International Nuclear Event Scale
The International Nuclear Event Scale (INES) is a tool to promptly and consistently communicate to the public the safety significance of reported events at nuclear installations. By putting events into proper perspective, the Scale can facilitate common understanding among the nuclear community, the media, and the public.
Events are classified on the Scale at seven levels. The lower levels (1-3) are termed incidents, and the upper levels (4-7) accidents. Events which have no safety significance are classified as level zero, or below scale, and are termed "deviations." Events which have no safety relevance are termed "out of scale."
Level 1 - Anomaly
Anomaly beyond the authorized 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".
Level 2 - Incident
Incidents with significant failure in safety provisions but with sufficient defense 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.
Level 3 - Serious Incident
External release of radioactivity above authorized 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. Example: Vandellos nuclear power plant, Spain, 1989.
Level 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 to the nuclear facility. Such an accident might include damage to the 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. Examples: Windscale Reprocessing Plant, United Kingdom, 1973, Saint-Laurent Nuclear Power Plant, France, 1980.
Irradiation of one or more workers which result in an overexposure where a high probability of early death occurs. Example: Buenos Aires Critical Assembly, Argentina, 1983.
Level 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. Example: Windscale Pile, United Kingdom, 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. Example: Three Mile Island Nuclear Power Plant, United States of America, 1979.
Level 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. Example: Kyshtym Reprocessing Plant, Union of Soviet Socialist Republics (now in Russia), 1957.
Level 7 - Major Accident
External release of a large fraction of the radioactive material in a large facility, e.g., 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; long-term environmental consequences. Example: Chernobyl Nuclear Power Plant, Union of Soviet Socialist Republics (now in Ukraine), 1986.
1 Sievert = 100 Rem
1 Becquerel = 1 disintegration/second
1 Curie = 3.7E10 disintegrations/second