A scale by which one can classify safety significance of incidents at nuclear installations. Designed by the International Atomic Energy Agency (IAEA) and Nuclear Energy Agency (NEA). The purpose is to provide the general public with a way to put the incident in context with other reported incidents. Currently a little over 30 countries have agreed to use this scale when reporting incidents. 

Events can be classified in seven different levels, and are either termed accidents or incidents. There's also a below scale level 0, called deviation, for events with no safety significance. If an event has no safety relevance, it is off scale. Each level has a set of criteria in the categories of off-site impact, on-site impact and defence in depth degradation. The "defence in depth" refers to the different safety layers that the nuclear installation has and all of these have to fail before the event is classified Accident. See further down for the different criteria. 

 

The levels

  Classification                INES level
|------------------------------------------------------------------------------|
| Accident                      7 Major Accident
| 
|                               6 Serious Accident
|
|                               5 Accident with off-site risk
|
|                               4 Accident without significant off-site risk
|
| -----------------------------------------------------------------------------|
| Incident                      3 Serious incident
|
|                               2 Incident
| 
|                               1 Anomaly
| 
| -----------------------------------------------------------------------------|
| Deviation                     0 No safety significance                 
|
|------------------------------------------------------------------------------|
 

Examples of previously classified events:

 

Description of the criteria for classification:

  • Level 7
    • "External release of a large fraction of the radioactive material in a large facility (e.g. the core of a power reactor). This would typically involve a mixture of short and long-lived radioactive fission products (in quantities radiologically equivalent to more than tens of thousands terabecqerels of iodine-131). Such a release would result in the possibility of acute health effects; delayed health effects over a wide area, possibly involving more than one country; long-term environmental consequences."
  • Level 6
    • "External release of radioactive material (in quantities radiologically equivalent to the order of thousands to tens of thousands of terabecquerels of iodine-131). Such a release would be likely to result in full implementation of countermeasures covered by local emergency plans to limit serious health effects."
  • Level 5
    • "External release of radioactive material (in quantities radiologically equivalent to the order of hundreds to thousands of terabecquerels of iodine-131). Such a release would be likely to result in partial implementation of countermeasures covered by emergency plans to lessen the likelihood of health effects."
    • "Severe damage to the nuclear facility. This may involve severe damage to a large fraction of the core of a power reactor, a major criticality accident or a major fire or explosion releasing large quantities of radioactivity within the installation."
  • Level 4
    • "External release of radioactivity resulting in a dose to the most exposed individual off-site of the order of a few milisieverts. With such a release the need for off-site protective actions would be generally unlikely except possibly for local food control."
    • "Significant damage to the nuclear facility. Such an accident might include damage to nuclear plant leading to major on-site recovery problems such as partial core melt in a power reactor and comparable events at non-reactor installations."
    • "Irradiation of one or more workers which result in an overexposure where a high probability of early death occurs."
  • Level 3
    • "External release of radioactivity above authorized limits, resulting in a dose to the most exposed individual off site of the order of tenths of millisievert. With such a release, off-site protective measures may not be needed."
    • "On-site events resulting in doses to workers sufficient to cause acute health effects and/or an event resulting in a severe spread of contamination for example a few thousand terabecquerels of activity released in a secondary containment where the material can be returned to a satisfactory storage area."
    • "Incidents in which a further failure of safety systems could lead to accident conditions, or a situation in which safety systems would be unable to prevent an accident if certain initiators were to occur."
  • Level 2
    • "Incidents with significant failure in safety provisions but with sufficient defence in depth remaining to cope with additional failures."
    • "An event resulting in a dose to a worker exceeding a statutory annual dose limit and/or an event which leads to the presence of significant quantities of radioactivity in the installation in areas not expected by design and which require corrective action."
  • Level 1
    • "Anomaly beyond the authorized operating regime. This may be due to equipment failure, human error or procedural inadequacies. (Such anomalies should be distinguished from situations where operational limits and conditions are not exceeded and which are properly managed in accordance with adequate procedures. These are typically 'below scale')."

 

Source: IAEA. The descriptions of the criteria are copyrighted to IAEA and are used with permission.

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