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.
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.
Impact on people and environment
Major release of radioactive 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 an eventual death toll of 56 at the time of the UNSCEAR report, as well as a projected 4,000 additional cancer fatalities (official WHO estimate) that will eventually occur among the people exposed to the highest doses of radiation. As a result of the plumes of radio-isotopes, the city of Chernobyl (pop. 14,000) was largely abandoned, the larger city of Pripyat (pop. 49,400) was completely abandoned, and a 30 kilometres (19 mi) exclusion zone around the reactor was established.
Fukushima Daiichi nuclear disaster, a series of events beginning on 11 March 2011. A month later the Japanese government's nuclear safety agency rated it level 7. 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. A temporary exclusion zone of 20 kilometres (12 mi) was established around the plant as well as a 30 kilometres (19 mi) voluntary evacuation zone; In addition, the evacuation of Tokyo – Japan's capital and the world's most populous metropolitan area, 225 kilometres (140 mi) away – was at one point considered.
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 Chemical Combine (MCC) Soviet Union, 29 September 1957. A failed cooling system at a military nuclear waste reprocessing facility caused an explosion with a force equivalent to 70-100 tons of TNT. About 70 to 80 metric tons of highly radioactive material were carried into the surrounding environment. The impact on the local population is not fully known, however reports of a unique condition known as Chronic radiation syndrome is reported due to the moderately high dose rates that 66 locals were continually exposed to. At least 22 villages were evacuated.
Impact on people and environment
Limited release of radioactive material likely to require implementation 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. 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. A combination of design and operator errors caused a gradual loss of coolant, leading to a partial meltdown. An unknown amount of radioactive gases were released into the atmosphere, so injuries and illnesses that have been attributed to this accident can be deduced from epidemiological studies but can never be proven.
First Chalk River accident, 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.
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 from 1955 to 1979.
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 on research reactor RA-2 during fuel rod rearrangement killed one operator and injured two 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.
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.
Davis-Besse Nuclear Power Station (United States), 2002; negligent inspections resulted in corrosion through 6 inches (15.24 cm) of the carbon steel reactor head leaving only 3⁄8 inch (9.5 mm) of stainless steel cladding holding back the high-pressure (~2500 psi, 17 MPa) reactor coolant.
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.
Fukushima Daini Nuclear Power Plant Mars 2011, following the 2011 Tōhoku earthquake.
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.
(Arrangements for reporting minor events to the public differ from country to country. It is difficult to ensure precise consistency in rating events between INES Level-1 and Below scale/Level-0)
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 18,000 litres (4,000 imp gal; 4,800 US gal) of water containing 75 kilograms (165 lb) of unenriched uranium into the environment.
No safety significance.
Examples:
4 June 2008: Krško, Slovenia: Leakage from the primary cooling circuit.
17 December 2006, Atucha, Argentina: Reactor shutdown due to tritium increase in reactor compartment.
13 February 2006: Fire in Nuclear Waste Volume Reduction Facilities of the Japanese Atomic Energy Agency (JAEA) in Tokaimura.
There are also events of no safety relevance, characterized as "out of scale".
Examples:
17 November 2002, Natural Uranium Oxide Fuel Plant at the Nuclear Fuel Complex in Hyderabad, India: A chemical explosion at a fuel fabrication facility.
29 September 1999: H.B. Robinson, United States: A tornado sighting within the protected area of the nuclear power plant.
5 March 1999: San Onofre, United States: Discovery of suspicious item, originally thought to be a bomb, in nuclear power plant.
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) to address these issues. One example given, is that it would deal with what some regard as having the potential to cause confusion, such as the underground and contained Lucens reactor meltdown presently residing in same event category as the Three Mile Island accident, which in contrast, did cause releases to the public.
One study found that the INES scale of the IAEA is highly inconsistent, and the scores provided by the IAEA incomplete, with many events not having an INES rating. Further, the actual accident damage values do not reflect the INES scores. For example, the Fukushima disaster should have an INES level of 10 or 11, rather than the top level of 7. A quantifiable, continuous scale might be preferable to the INES, in the same way that the antiquated Mercalli scale for earthquake magnitudes was superseded by the continuous physically-based Richter scale. When such a framework is established, data on nuclear incidents and accidents can made more rigorous, and transparent, accident risks can be better understood, and perhaps even minimised.
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.
