The INES scale

The INES scale (International Nuclear and radiological Event Scale) was developed, starting in 1989, by IAEA, the International Atomic Energy Agency, with the scope of classifying nuclear and radiological accidents and to make the public immediately aware of the gravity of each accident. The INES scale consists of 7 levels plus  Level 0 under this scale, and it is subdivided into two parts: accidents  (from Level 7 to Level 4), i.e. all the events that produce significant damages to people, the environment or to things, and incidents (from Level 3 to Level 1), in other words events that produce damages that are not very significant to people, the environment and to things. Level 0 is classified as a deviation. It is a logarithmic scale and the difference between one level and the next  therefore, amounts to a ten-fold increase in the damages.
Level 7 – major accident
Major release, outside a large power plant, of radioactive material, over a very vast area, with consequent severe effects on the health of the population that is exposed, and severe consequences on the environment.
Examples:
The disaster at Chernobyl, Ukraine, 1986. Overheating, up to the fusion of the core of a poorly protected nuclear reactor. Explosion (not nuclear) of the reactor and release of radioactive material  in the environment,
The disaster at Fukushima Dai-ichi (reactors 1, 2, 3) subsequent to the Sendai earthquake in March 2011, initially classified level 4, and subsequently, as the weeks passed, classified level 5, and over one month after the accident, after large losses of radioactivity, classified level 7.
Level 6 – serious accident
Significant release of radioactive material outside. The radiological equivalent amount of 1 to 10PBq iodine-131, which requires the complete implementation of planned countermeasures, which are part of an external emergency plan in order to limit the severe effects on the health of the population.
Examples:
The accident in Kyshtym, Mayak, Russia, 1957. The breakdown of the cooling system of a nuclear fuel reprocessing plant, overheating and explosion (not nuclear) of the deposit involving release  of radioactive material in the environment.
Level 5 – accident with off-site risk
Release, outside, of radioactive material – a radiological equivalent amount of 100 to 1000 TBq, requiring partial implementation of planned countermeasures. Severe damage to the reactor core or the protective barriers.
Examples:
The accident at Three Mile Island, United States, 1979. Severe damage to the reactor core and the radiation protection barriers.
Accident in Goiânia, Brazil, 1987. Radioactive contamination due to theft of radiotherapy equipment from an abandoned hospital.
Level 4 – accident without significant  off-site risk
Accident with minor external impact, with radiological exposure of the surrounding population within the prescribed limits. Significant damage to the reactor core or the protective barriers. Exposure of a worker at the plant with fatal consequences.
Examples:
Accident at the Windscale (currently Sellafield) reprocessing plant, United Kingdom, 1973.
Accident at the Saint-Laurent nuclear power plant, France, 1980.
Level 3 – serious incident
Event with a very mild impact outside, with radiological exposure of the population in the surrounding area below the prescribed limits. Severe contamination in the plant and/or severe consequences on the health of the workers at the plant.
Level 2 – incident
Event with no external impact. Significant internal contamination of the plant and/or overexposure of workers at the plant.
Level 1 – anomaly
Anomaly beyond the authorized operating regime .
Level 0 – deviation
Event without consequences on safety.

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