International Nuclear Event Scale
The International Nuclear and Radiological Event Scale (INES) was introduced in 1990 by the International Atomic Energy Agency (IAEA) in order to enable prompt communication of safety-significant information in case of nuclear accidents.The scale is intended to be logarithmic, similar to the moment magnitude scale that is used to describe the comparative magnitude of earthquakes. Each increasing level represents an accident approximately ten times more severe than the previous level. 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, the INES level of an incident is assigned well after the incident occurs. Therefore, the scale has a very limited ability to assist in disaster-aid deployment.
As INES ratings are not assigned by a central body, high-profile nuclear incidents are sometimes assigned INES ratings by the operator, by the formal body of the country, but also by scientific institutes, international authorities or other experts which may lead to confusion as to the actual severity.
Details
A number of criteria and indicators are defined to assure coherent reporting of nuclear events by different official authorities. There are seven nonzero levels on the INES scale: three incident-levels and four accident-levels. There is also a level 0.
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: Major accident
Impact on people and environment
Major release of radio¬active ¬material with widespread health and environmental effects requiring implementation of planned and extended ¬countermeasures
There have been two such events to date:
• Chernobyl disaster, 26 April 1986. A power surge during a test procedure resulted in a criticality accident, leading to a powerful steam explosion and fire that released a significant fraction of core material into the environment, resulting in a death toll of 56 as well as estimated 4,000 additional cancer fatalities (official WHO estimate) among people exposed to elevated doses of radiation. As a result, the city of Chernobyl (pop. 14,000) was largely abandoned, the larger city of Pripyat (pop. 49,400) was completely abandoned, and a permanent 30 km exclusion zone around the reactor was established.
• Fukushima Daiichi nuclear disaster, a series of events beginning on 11 March 2011. Rated level 7 on 11 April 2011 by the Japanese government's nuclear safety agency.[2][3] Major damage to the backup power and containment systems caused by the 2011 Tōhoku earthquake and tsunami resulted in overheating and leaking from some of the Fukushima I nuclear plant's reactors. Each reactor accident was rated separately; out of the six reactors, three were rated level 5, one was rated at a level 3, and the situation as a whole was rated level 7.[4] A temporary exclusion zone of 20 km was established around the plant as well as a 30 km voluntary evacuation zone;[5] in addition, the evacuation of Tokyo – Japan's capital and the world's most populous metropolitan area, 225 km away – was at one point considered, threatening the future of the Japanese state.[6] See also 2011 Japanese nuclear accidents.
See also: Comparison of Fukushima and Chernobyl nuclear accidents
Level 6: Serious accident
Impact on people and environment
Significant release of radioactive material likely to require implementation of planned countermeasures.
There has been only one such event to date:
• Kyshtym disaster at Mayak, 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: Accident with wider consequences
Impact on people and environment
Limited release of radioactive ¬material likely to require i¬mplementation of some planned¬ countermeasures.
Several deaths from ¬radiation.
Impact on radiological barriers and control
Severe damage to reactor core.
Release of large quantities of radioactive material within an installation with a high probability of significant public exposure. This could arise from a major criticality accident or fire.
Examples:
• Windscale fire (United Kingdom), 10 October 1957.[7] Annealing of graphite moderator at a military air-cooled reactor caused the graphite and the metallic uranium fuel to catch fire, releasing radioactive pile material as dust into the environment.
• Three Mile Island accident near Harrisburg, Pennsylvania (United States), 28 March 1979.[8] A combination of design and operator errors caused a gradual loss of coolant, leading to a partial meltdown. Radioactive gases were released into the atmosphere, no health risks have been attributed to this accident.
• First Chalk River accident,[9][10] Chalk River, Ontario (Canada), 12 December 1952. Reactor core damaged.
• Lucens partial core meltdown (Switzerland), 21 January 1969. A test reactor built in an underground cavern suffered a loss-of-coolant accident during a startup, leading to a partial core meltdown and massive radioactive contamination of the cavern, which was then sealed.
• Goiânia accident (Brazil), 13 September 1987. An unsecured caesium chloride radiation source left in an abandoned hospital was recovered by scavenger thieves unaware of its nature and sold at a scrapyard. 249 people were contaminated and 4 died.
Level 4: Accident with local consequences
Impact on people and environment
Minor release of radioactive material unlikely to result in implementation of planned countermeasures other than local food controls.
At least one death from radiation.
Impact on radiological barriers and control
Fuel melt or damage to fuel ¬resulting in more than 0.1% release of core inventory.
Release of significant quantities of radioactive material within an installation with a high ¬probability of significant public exposure.
Examples:
• Sellafield (United Kingdom) – five incidents 1955 to 1979[11]
• SL-1 Experimental Power Station (United States) – 1961, reactor reached prompt criticality, killing three operators.
• Saint-Laurent Nuclear Power Plant (France) – 1969, partial core meltdown; 1980, graphite overheating.
• 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: Serious incident
Impact on people and environment
Exposure in excess of ten times the statutory annual limit for workers.
Non-lethal deterministic health effect (e.g., burns) from radiation.
Impact on radiological barriers and control
Exposure rates of more than 1 Sv/h in an operating area.
Severe contamination in an area not expected by design, with a low probability of ¬significant public exposure.
Impact on defence-in-depth
Near accident at a nuclear power plant with no safety provisions remaining.
Lost or stolen highly radioactive sealed source.
Misdelivered highly radioactive sealed source without adequate procedures in place to handle it.
Examples:
• THORP plant Sellafield (United Kingdom) – 2005.
• Paks Nuclear Power Plant (Hungary), 2003; fuel rod damage in cleaning tank.
• Vandellos Nuclear Power Plant (Spain), 1989; fire destroyed many control systems; the reactor was shut down.
• San Onofre Nuclear Generating Station (United States), 2011; Ammonia leak. No evacuation called for.
Level 2: Incident
Impact on people and environment
Exposure of a member of the public in excess of 10 mSv.
Exposure of a worker in excess of the statutory annual limits.
Impact on radiological barriers and control
Radiation levels in an operating area of more than 50 mSv/h.
Significant contamination within the facility into an area not expected by design.
Impact on defence-in-depth
Significant failures in safety ¬provisions but with no actual ¬consequences.
Found highly radioactive sealed orphan source, device or transport package with safety provisions intact.
Inadequate packaging of a highly radioactive sealed source.
Examples:
• Blayais Nuclear Power Plant flood (France) December 1999
• Ascó Nuclear Power Plant (Spain) April 2008; radioactive contamination.
• Forsmark Nuclear Power Plant (Sweden) July 2006; backup generator failure; two were online but fault could have caused all four to fail.
• Gundremmingen Nuclear Power Plant (Germany) 1977; weather caused short-circuit of high-tension power lines and rapid shutdown of reactor
• Shika Nuclear Power Plant (Japan) 1999; criticality incident caused by dropped control rods, covered up until 2007
Level 1: Anomaly
Impact on defence-in-depth
Overexposure of a member of the public in excess of statutory ¬annual limits.
Minor problems with safety components with significant defence-in-depth remaining.
Low activity lost or stolen radioactive source, device or transport package.
Examples:
• Penly (Seine-Maritime, France) 5 April 2012; an abnormal leak on the primary circuit of the reactor n°2 was found in the evening of 5 April 2012 after a fire in reactor n°2 around noon was extinguished.
• Gravelines (Nord, France), 8 August 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.
• TNPC (Drôme, France), July 2008; leak of 6,000 litres (1,300 imp gal; 1,600 US gal) of water containing 75 kilograms (170 lb) of uranium into the environment.
Criticism
Deficiencies in the existing INES have emerged through comparisons between the 1986 Chernobyl disaster and 2011 Fukushima nuclear disaster.
• Firstly, the scale is essentially a discrete qualitative ranking, not defined beyond event level 7.
• Secondly, it was designed as a public relations tool, not an objective scientific scale.
• Thirdly, its most serious shortcoming is that it conflates magnitude with intensity. David Smythe has proposed a new quantitative nuclear accident magnitude scale (NAMS).[24]
Nuclear experts say that the "INES emergency scale is very likely to be revisited" given the confusing way in which it was used in the 2011 Japanese nuclear accidents.
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