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 significance information in case of nuclear accidents.

The scale is intended to be logarithmic, similar to the Richter 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.

Commonly, the organisation where the nuclear incident occurs assigns a first provisional INES rating to an incident, after it is being reviewed and possibly revised by the designated national radiation authority.

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. 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

The only accident:

• 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 and a permanent loss of large areas of habitable land. The disaster is the only Level 7 Event that has ever occurred.

Level 6: Serious accident

Impact on People and Environment

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

Example

• 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 i­mplementation of some planned­ countermeasures.

Several deaths from ­radiation.

Example

• 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.

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.

Example: Three Mile Island accident (Harrisburg, United States), 28 March 1979. 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.

Other examples:

• First Chalk River Accident Chalk River, Ontario, Canada, 12 December 1952. Reactor core damaged.
• 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 the 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) – 5 incidents 1955 to 1979
• 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.
• Fukushima I Nuclear Power Plant (Japan) – 2011, reactor shutdown after the 2011 Sendai earthquake and tsunami, failure of emergency cooling caused an explosion.

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 (fuel rod damage in cleaning tank) (Hungary) – 2003.
• Vandellos Nuclear Power Plant, Spain (A fire destroyed many control systems; the reactor was shut down) – 1989.

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:

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

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)

Examples:

• 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.

Level 0: Deviation

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

Out of Scale

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
• 4 November 1999: H.B. Robinson, United States: A Tornado sighting within the protected area of the NPP.
• 15 April 1999: San Onofre, United States: Discovery of suspicious item in nuclear power plant