The International Nuclear Event Scale (INES) classifies civil nuclear events according to their radiological severity. The least serious, from 0 to 3, are called incidents, while the most serious, from 4 to 7, are calledaccidents. Only two events merited a ranking of 7: the Chernobyl and Fukushima accidents.
It covers events in nuclear power plants, fuel transport and research.
Severity from 0 to 3: nuclear incidents
There are 4 levels of nuclear incidents:
- Level 0, “deviation”: this is a simple deviation, an anomaly with no impact on safety. In France, there are around 1,000 such incidents a year.
- Level 1, “anomaly”: an anomaly in authorized operation. There is no concrete negative effect. In France, there are around a hundred of these every year.
- Level 2, “incident”: significant contamination occurs on site, or a worker is overexposed. There are a few of these a year. One example is the flooding of the Blayais nuclear power plant in 1999.
- Level 3, “serious incident”: contamination is severe or a worker suffers acute effects as a result of exposure. There is very little release to the outside world, and an accident is narrowly averted. An example is the Forback incident, in which 3 employees entered a gas pedal and were irradiated.
All nuclear incidents are reported by the operator to the French Nuclear Safety Authority (ASN). Only level 0 incidents are not systematically made public. From level 1 upwards, they are the subject of a press release.
Severity levels 4 to 7: nuclear accidents
Nuclear accidents are all characterized by the loss of defenses in depth.
- Level 4, “accident with no significant risk outside the site”: the reactor or radiological barriers are damaged, or a worker is fatally exposed; there is a minor release, which exposes the public without exceeding health limits. An example is the damage to a core at the Saint-Laurent-A2 power plant in 1980. This is the maximum level reached in France.
From level 5 upwards, the event involves serious damage to the reactor or radiological barriers.
- Level 5, “accident involving an off-site risk”. There is a limited release of radioactivity, but this may justify measures such as evacuation. The best-known example is the Three Miles Island accident in the USA in 1970.
- Level 6, “severe accident”. The release of radioactivity is significant and may justify radical countermeasures. The Kyshtym disaster in the USSR in 1957 is just one example.
- Level 7, “major accident”, is the maximum level: the release is considerable and has a widespread effect on health and the environment. There have been two such accidents: Chernobyl and Fukushima.
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