International Nuclear Event Scale

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The International Nuclear Event Scale (INES) was introduced by the International Atomic Energy Agency (IAEA) in order to enable prompt communication of safety significance information in case of nuclear accidents. A number of criteria and indicators are defined to assure coherent reporting of nuclear events by different official authorities. There are 7 levels on the INES scale:

7
Major accident
(maximum credible accident)

6
Serious accident

5
Accident with off-site risk

4
Accident without off-site risk

3
Serious incident

2
Incident

1
Anomaly

0
Deviation, no safety relevance

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Details

Level 7 is characterized by large off-site impact, widespread health and environmental effects. Example: Chernobyl accident (former Soviet Union) - 1986.

Level 6 : significant off-site release, likely to require full implementation of planned countermeasures. Example: Mayak (former Soviet Union)- 1957.

Level 5 : limited off-site release, likely to require partial implementation of planned countermeasures. Examples: Windscale fire (United Kingdom) - 1957, Three Mile Island (United States) - 1979.

The Levels 5-7 are related to severe damage of the reactor core and the radiological barriers.

Level 4 is related to significant damage of the reactor core / radiological barriers and/or a fatal exposure of a worker (or more), but the off-site impact is minor, resulting in public exposure of the order of the prescribed limits. Examples: Windscale (United Kingdom) - 1973, Saint-Laurent (France) - 1980, Buenos Aires (Argentina) - 1983.

Level 3 is characterized by very small off-site impact, although related to severe spread of contamination on-site / acute health effects to a worker (or more). It is a "near accident" event, when no safety layers are remaining. Example: Vandellos (Spain) - 1989, THORP plant Sellafield (United Kingdom) - 2005.

Level 2 is an incident with no off-site impact, related to significant spread of contamination on-site / overexposure of a worker.

Level 1 is an anomaly beyond the authorized operating regime.

Level 0 is a "below-scale event" of no safety significance.

There are also events of no safety relevance, characterized as "out of scale".

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