International Nuclear Event Scale

International Nuclear Event Scale

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 significance 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.



INES en.svg

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 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 a death toll of 56 as well as estimated 4,000 additional cancer fatalities 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 30 km exclusion zone 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] An exclusion zone of 20 km was established around the plant as well as a 30 km voluntary evacuation zone.[5] See also 2011 Japanese 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. This is the only accident to go over 5 on the scale besides Chernobyl and Fukushima.[6]

Level 5: Accident with wider consequences

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.

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.

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.

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.

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.

(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)

  • 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.[12]
  • 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.

Level 0: Deviation

No safety significance.

  • 4 June 2008: Krško, Slovenia: Leakage from the primary cooling circuit.[13]
  • 17 December 2006, Atucha, Argentina: Reactor shutdown due to tritium increase in reactor compartment.[14]
  • 13 February 2006: Fire in Nuclear Waste Volume Reduction Facilities of the Japanese Atomic Energy Agency (JAEA) in Tokaimura.[15]

Out of scale

There are also events of no safety relevance, characterized as "out of scale".[16]

  • 17 November 2002, Natural Uranium Oxide Fuel Plant at the Nuclear Fuel Complex in Hyderabad, India: A chemical explosion at a fuel fabrication facility.[17]
  • 29 September 1999: H.B. Robinson, United States: A tornado sighting within the protected area of the nuclear power plant (NPP).[18][19][20]
  • 5 March 1999: San Onofre, United States: Discovery of suspicious item, originally thought to be a bomb, in nuclear power plant.[21]

See also


  1. ^ "Event scale revised for further clarity". 6 October 2008. Retrieved 13 September 2010. 
  2. ^ "Japan to raise Fukushima crisis level to worst". Retrieved 12 April 2011. 
  3. ^ "Japan raises nuclear crisis to same level as Chernobyl". Reuters. 12 April 2011. 
  4. ^ "Japan: Nuclear crisis raised to Chernobyl level". BBC News. 12 April 2011. Retrieved 12 April 2011. 
  5. ^ "Japan's government downgrades its outlook for growth". BBC News. 13 April 2011. Retrieved 13 April 2011.  The death toll rose to over 15,000 with 8,206 missing and 5,363 injured the numbers are still rising.
  6. ^
  7. ^ Richard Black (18 March 2011). "Fukushima - disaster or distraction?". BBC. Retrieved 7 April 2011. 
  8. ^ Spiegelberg-Planer, Rejane. "A Matter of Degree". IAEA Bulletin. IAEA. Retrieved 16 March 2011. 
  9. ^ Canadian Nuclear Society (1989) The NRX Incident by Peter Jedicke
  10. ^ The Canadian Nuclear FAQ What are the details of the accident at Chalk River's NRX reactor in 1952?
  11. ^ G A M Webb et al. (March 2006). "Classification of events with an off-site radiological impact at the Sellafield site between 1950 and 2000, using the International Nuclear Event Scale". Journal of Radiological Protection 26 (1): 33. Bibcode 2006JRP....26...33W. doi:10.1088/0952-4746/26/1/002. PMID 16522943. 
  12. ^ (AFP) – 10 août 2009. "AFP: Incident "significatif" à la centrale nucléaire de Gravelines, dans le Nord". Retrieved 13 September 2010. 
  13. ^ News | Slovenian Nuclear Safety Administration
  14. ^ (Spanish)
  15. ^ (Japanese)
  16. ^ IAEA: "This event is rated as out of scale in accordance with Part I-1.3 of the 1998 Draft INES Users Manual, as it did not involve any possible radiological hazard and did not affect the safety layers."
  17. ^
  18. ^ "NRC: SECY-01-0071 – Expanded NRC Participation in the Use of the International Nuclear Event Scale" (pdf). US Nuclear Regulatory Commission. 25 April 2001. pp. 8. Retrieved 13 March 2011. [dead link]
  19. ^ "SECY-01-0071-Attachment 5 - INES Reports, 1995-2000" (pdf). US Nuclear Regulatory Commission. 25 April 2001. pp. 1. Retrieved 13 March 2011. [dead link]
  20. ^
  21. ^

External links

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