International Nuclear Event Scale

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Impression of the 1986 exploded Chernobyl NPP unit 4

After the Chernobyl accident, the International Nuclear Event Scale (INES) has enhanced reporting on events at nuclear power plants[1]. It was implemented in 1990[2]. The UN's International Atomic Energy Agency (IAEA) describes INES as a tool for "promptly communicating to the public in consistent terms" what a nuclear episode means[2]. It has three factors: radioactivity releases to the public; barriers against radiation at a nuclear site; and civil-defence measures[2].

In the INES the events are roughly divided into deviations, incidents and accidents[1]. The events are classified on the Scale at seven levels[1]. Each increase in level on the scale indicates a roughly 10-fold increase in severity[2].

The lower levels (1-3) are termed incidents[1]. The upper levels (4-7) are termed accidents[1]. The events which have no safety significance are classified as level 0/below scale[1]. They are termed deviations[2][1]. The lower levels consists of anomalies, incidents and serious incidents[1]. The upper levels are comprised of accidents without significant off-site risk, accidents with off-site risk, serious accidents and major accidents[1].

Description of INES levels

  • INES 0: Events known as "deviations" that have no safety significance[2]
  • INES 1: Anomaly[2]
Minor problem with safety components at a nuclear facility, but significant safety margin remaining
  • INES 2: Incident[2]
Radiation levels in an operating area of a nuclear facility of more than 50 millisieverts (mSv) per hour. Exposure of a member of the public to radiation in excess of 10 mSv, exposure of a worker in excess of statutory annual limits.
  • INES 3: Serious Incident[2]
Severe contamination in an area of a facility, with non-lethal injuries such as radiation burns. Low probability of significant public exposure.
  • INES 4: Accident with local consequences[2]
Partial meltdown or damage to fuel, release of significant quantities of radioactive material within an installation. No counter-measures likely to be needed other than local food controls.
  • INES 5: Accident with wider consequences[2]
Severe damage to reactor core, large quantities of radioactive material released within a site. Limited release of material to the wider environment, requiring implementation of some planned countermeasures.
  • INES 6: Serious accident[2]
Significant release of radioactive material likely to require implementation of planned countermeasures.
  • INES 7: Major accident[2]
Major release of radioactive material with widespread health and environmental effects, requiring implementation of planned and extended countermeasures.


Examples of INES accidents

INES 7:

  • 1986 Chernobyl (Pripyat) (UA)[2] - meltdown and explosion
  • 2011 Fukushima (J)[3] - several meltdowns and explosions

INES 6:

  • 1957 Mayak (Kyshtym) (RUS) - explosion at a waste tank[2]

INES 5:

  • 1957 Windscale (UK) - fire nuclear plant[2]
  • 1979 Three Mile Island (Harrisburg) (USA) - meltdown[2]


Other concepts

In Germany accidents up to INES level 4 are called GAU (German for größter anzunehmender Unfall) refering to the US-American concept of the maximum credible accident a nuclear plant needed to be prepared for. Accidents of higher level are called Super-GAU as their impacts are even bigger than the biggest accidents the plant concepts necessarily have to deal with.[4].


Atomic catastrophe information network

The IAEA maintains an information exchange network between the countries participating in the use of the Scale. Reports on the events belonging to the level 2 or above it are submitted to the IAEA through the NEWS (Nuclear Event Web-based System). The IAEA should have information on the level of the event in its disposal within 24 hours.[1]

The event level is defined in the country where the event takes place.[1]


Further information =