International Nuclear Event Scale

5 min readApr 30, 2021

The International Nuclear and Radiological Event Scale (INES) was introduced in 1990[1] 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 as severe as 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 this subjectivity the INES level of an incident is assigned well after the fact. The scale is therefore intended to assist in disaster-aid deployment.

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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 defense in depth degradation.

LevelClassificationDescriptionExamples7Major accidentImpact on people and environment:

Major release of radioactive material with widespread health and environmental effects requiring implementation of planned and extended countermeasures.

To date, there have been two Level 7 accidents:

Chernobyl disaster, 26 April 1986. Unsafe conditions 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 4,000–27,000.[2][3][4][5][6] As a result of the plumes of radioisotopes, 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. 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.[7] A temporary exclusion zone of 20 kilometres (12 mi) was established around the plant,[8] and officials considered evacuating Tokyo, Japan’s capital and the world’s most populous metropolitan area, 225 kilometres (140 mi) away.[9] (Initially rated as Level 5, upgraded later to 7.)[10][11]

6Serious accidentImpact on people and environment:

Significant release of radioactive material likely to require implementation of planned countermeasures.

To date, there has been one Level 6 accident:

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.[12] 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.[13]

5Accident with wider consequencesImpact 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.
First Chalk River accident,[14][15] Chalk River, Ontario (Canada), 12 December 1952. Reactor core damaged.
Windscale fire at Sellafield (United Kingdom), 10 October 1957.[16] 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. 100 to 240 cancer deaths were caused by the incident.[17][18][19]
Three Mile Island accident near Harrisburg, Pennsylvania (United States), 28 March 1979.[20] A combination of design and operator errors caused a gradual loss of coolant, leading to a partial meltdown. The amounts of radioactive gases released into the atmosphere are still not known, so injuries and illnesses that have been attributed to this accident can only be deduced from epidemiological studies.
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.[13]

4Accident with local consequencesImpact 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.
Sellafield (United Kingdom) — five incidents from 1955 to 1979.[21]
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.
Lucens reactor (Switzerland) — 1969, blocked coolant channel caused fuel assembly to melt and catch fire, no radiation exposure to staff or public
Jaslovské Bohunice (Czechoslovakia) — 1977, partial core meltdown resulted in minor release of radiation to reactor building.
Andreev Bay nuclear accident (Soviet Union) — 1982, a spent nuclear fuel storage facility was damaged and caused approximately 700,000 tonnes (770,000 tons) of highly radioactive water to leak into the Barents Sea.[22][23]
Buenos Aires (Argentina) — 1983, criticality accident on research reactor RA-2 during fuel rod rearrangement killed one operator and injured two others.
Tokaimura nuclear accident (Japan) — 1999, three inexperienced operators at a reprocessing facility caused a criticality accident; two of them died.[13]
Mayapuri (India) — 2010, a university irradiator was sold for scrap and dismantled by dealers unaware of the hazardous materials.

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International Nuclear Event Scale

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