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The International Nuclear Event Scale (INES) was introduced in 1990^[1] by the International Atomic Energy Agency (IAEA) in order to enable prompt communication of safety significance information in case of nuclear accidents.

The scale is inspired by the Richter scale that is used to describe the comparative magnitude of earthquakes. Each increasing level represents an accident ten times more severe than the previous level. However, compared to earthquakes, where the event intensity can be quantitatively evaluated, the level of severity of a man-made disaster, such as a nuclear accident, is more subject to interpretation. Because of the difficulty of interpreting these data, the INES level of an incident is frequently assigned well after the incident occurs. Therefore, the scale has a very limited ability to assist in disaster-aid deployment.

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; 3 incident-levels and 4 accident-levels

Details

The level on the scale is determined by the highest of three scores: Off site effects, on site effects, and Defence in depth degradation.

Level 7

A large off-site impact with widespread health and environmental effects.

Only example: Chernobyl disaster, 26 April 1986. An uncontrolled power excursion in a reactor without a containment building caused a powerful steam explosion and fire that released a significant fraction of core material into the environment, resulting in a death toll of 57 as well as estimated 4,000 additional cancer fatalities among 600,000 people exposed to elevated doses of radiation.^[2]

Level 6

Significant off-site release, likely to require full implementation of planned countermeasures.

Only example: Kyshtym disaster at Mayak (former Soviet Union), 29 September 1957. A failed cooling system at a military nuclear waste reprocessing facility caused a steam explosion that released 70-80 tons of highly radioactive material into the environment. Impact on local population is not fully known.

Level 5

Limited off-site release, likely to require partial implementation of planned countermeasures.

or

Severe damage to a reactor core/radiological barriers.

So far three events were classified as level 5:

• Windscale fire (United Kingdom), 10 October 1957. Annealing of graphite moderator at a military air-cooled reactor caused the graphite and the metallic uranium fuel to catch fire, releasing some of the core material as dust into the environment. No immediate casualties were recorded. Based on radionuclide release, 200-240 additional thyroid cancer cases were expected^{[citation needed]}.
• Three Mile Island accident (United States), 28 March 1979. A combination of design and operator errors caused a gradual loss of coolant, leading to a core meltdown. Some radioactive gases were released into the atmosphere in a controlled manner. At most 1 additional cancer fatality is expected.
• Goiânia accident (Brazil), 13 September 1987. An unsecured caesium chloride radiation source left in an abandoned hospital was recovered by squatters unaware of its nature and sold at a scrapyard. 249 people were contaminated and 4 died.

Level 4

Minor off-site impact resulting in public exposure of the order of the prescribed limits.

or

Significant damage to a reactor core/radiological barriers or the fatal exposure of a worker.

Examples:

• Sellafield (United Kingdom) - 5 incidents 1955 to 1979^[3]
• SL-1 Experimental Power Station (United States) - 1961, reactor reached prompt criticality, killing three operators
• Saint-Laurent Nuclear Power Plant (France) - 1980, partial core meltdown
• Buenos Aires (Argentina) - 1983, criticality accident during fuel rod rearrangement killed one operator and injured 2 others
• Jaslovské Bohunice (Czechoslovakia) - 1977, contamination of reactor building
• Tokaimura nuclear accident (Japan) - 1999, three inexperienced operators at a reprocessing facility caused a criticality accident; two of them died

Level 3

A very small off-site impact, public exposure at levels below the prescribed limits.

or

Severe spread of contamination on-site and/or acute health effects to one or more workers.

or

It is a "near accident" event, when no safety layers are remaining.

Examples:

• THORP plant Sellafield (United Kingdom) - 2005.
• Paks Nuclear Power Plant (fuel rod damage in cleaning tank) (Hungary) - 2003.

Level 2

This is an incident with no off-site impact, a significant spread of contamination on-site may have occurred.

or

Overexposure of a worker.

or

Incidents with significant failures in safety provisions.

Examples:

• Ascó Nuclear Power Plant, (Catalonia, Spain) April 2008; radioactive contamination
• Forsmark Nuclear Power Plant (Sweden); backup generator failure.

Level 1

This is an anomaly beyond the authorized operating regime.

Example:

• Gravelines (Nord, France), August 8, 2009; during the annual fuel bundle exchange in reactor #1, a fuel bundle snagged on to the internal structure. Operations were stopped, the reactor building was evacuated and isolated in accordance with operating procedures. ^[4]
• SOCATRI (Drôme, France), July 2008; leak of 6000 litres of water containing 75 kg of Uranium into the environment.

Level 0

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

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

See also

References

External links

• Nuclear Events Web-based System (NEWS), IAEA
• International Nuclear Event Scale factsheet, IAEA
• International Nuclear Event Scale, User's manual, IAEA, 2001