The International Nuclear Event Scale

General description of the scale | Using the Scale | Examples of classified nuclear events | Basic structure of the scale | The INES Scale (with criteria and examples)

General description of the scale

The International Nuclear Event Scale (INES) is a tool to promptly and consistently communicate to the public the safety significance of reported events at nuclear installations. By putting events into proper perspective, the Scale can ease common understanding among the nuclear community, the media, and the public. It was designed by an international group of experts convened jointly by the International Atomic Energy Agency (IAEA) and the Nuclear Energy Agency (NEA) of the Organisation for Economic Co-operation and Development. The group was guided in its work by the findings of a series of international meetings held to discuss general principles underlying such a scale. The Scale also reflects the experience gained from the use of similar scales in France and Japan as well as from consideration of possible scales in several other countries.

Initially applied for a trial period to classify events at nuclear power plants, 32 countries participated in the trial and international agencies and user countries monitored progress. The Scale operated successfully and now has been made available for formal adoption by each country. The Scale also has been extended and adapted to enable it to be applied to all nuclear installations associated with the civil nuclear industry and to any events occurring during the transport of radioactive materials to and from those facilities.

Events are classified on the Scale at seven levels. Their descriptors and criteria are shown opposite with examples of the classification of nuclear events which have occurred in the past at nuclear installations. The lower levels (1-3) are termed incidents, and the upper levels (4-7) accidents. Events which have no safety significance are classified as level 0/ below scale and are termed deviations. Events which have no safety relevance are termed out of scale.


The structure of the Scale is shown opposite, in the form of a matrix with key words. The words used are not intended to be precise or definitive. Each criterion is defined in detail within an INES Users' Manual. Events are considered in terms of three safety attributes or criteria represented by each of the columns: off-site impact, on-site impact, and defence in depth degradation.

The second column in the matrix relates to events resulting in off-site releases of radioactivity. Since this is the only consequence having a direct effect on the public, such releases are understandably of particular concern. Thus, the lowest point in this column represents a release giving the most exposed person off-site an estimated radiation dose numerically equivalent to about one-tenth of the annual dose limit for the public; this is classified as level 3. Such a dose is also typically about one-tenth of the average annual dose received from natural background radiation. The highest level is a major nuclear accident with widespread health and environmental consequences.

The third column considers the on-site impact of the event. This category covers a range from level 2 (contamination and/or overexposure of a worker) to level 5 (severe plant damage such as a core melt).

All nuclear facilities are designed so that a succession of safety layers act to prevent major on-site or off-site impact and the extent of the safety layers provided generally will be commensurate with the potential for on and off-site impact. These safety layers must all fail before substantial off-site or on-site consequences occur. The provision of these safety layers is termed "defence in depth". The fourth column of the matrix relates to incidents at nuclear installations or during the transportation of radioactive materials in which these defence in depth provisions have been degraded. This column spans the incident levels 1-3.

An event which has characteristics represented by more than one criterion is always classified at the highest level according to any one criterion.

Using the Scale

Examples of classified nuclear events




Basic structure of the scale

(Criteria given in matrix are broad indicators only)
Detailed definitions are provided in the INES users' manual

  CRITERIA OR SAFETY ATTRIBUTES
OFF-SITE IMPACT ON-SITE IMPACT DEFENCE IN DEPTH DEGRADATION
7
MAJOR ACCIDENT
MAJOR RELEASE:
WIDESPREAD HEALTH AND ENVIRONMENTAL EFFECTS
   
6
SERIOUS ACCIDENT
SIGNIFICANT RELEASE:
LIKELY TO REQUIRE FULL IMPLEMENTATION OF PLANNED COUNTERMEASURES
5
ACCIDENT WITH OFF-SITE RISK
LIMITED RELEASE:
LIKELY TO REQUIRE PARTIAL IMPLEMENTATION OF PLANNED COUNTERMEASURES
SEVERE DAMAGE TO REACTOR CORE/RADIOLOGICAL BARRIERS
4
ACCIDENT WITHOUT SIGNIFICANT OFF-SITE RISK
MINOR RELEASE:
PUBLIC EXPOSURE OF THE ORDER OF PRESCRIBED LIMITS
SIGNIFICANT DAMAGE TO REACTOR CORE/RADIOLOGICAL BARRIERS/FATAL EXPOSURE OF A WORKER
3
SERIOUS INCIDENT
VERY SMALL RELEASE:
PUBLIC EXPOSURE AT A FRACTION OF PRESCRIBED LIMITS
SEVERE SPREAD OF CONTAMINATION/ACUTE HEALTH EFFECTS TO A WORKER NEAR ACCIDENT- NO SAFETY LAYERS REMAINING
2
INCIDENT
  SIGNIFICANT SPREAD OF CONTAMINATION/OVEREXPOSURE OF A WORKER INCIDENTS WITH SIGNIFICANT FAILURES I N SAFETY PROVISIONS
1
ANOMALY
    ANOMALY BEYOND THE AUTHORISED OPERATING REGIME
0
BELOW SCALE EVENT DEVIATION
NO SAFETY SIGNIFICANCE

OUT OF SCALE EVENT NO SAFETY RELEVANCE





THE INTERNATIONAL NUCLEAR EVENT SCALE
for prompt communication of safety significance

LEVEL DESCRIPTOR CRITERIA EXAMPLES
ACCIDENTS
7
MAJOR ACCIDENT
  • 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 terabecquerels 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.
Chernobyl NPP, USSR (now Ukraine), 1986
6 SERIOUS ACCIDENT
  • 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.
Kyshtym Reprocessing Plant, USSR (now in Russia), 1957
5 ACCIDENT WITH OFF-SITE RISK
  • 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.
Windscale Pile, UK, 1957









Three Mile Island, USA, 1979

4 ACCIDENT WITHOUT SIGNIFICANT OFF-SITE RISK
  • 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.
Windscale Reprocessing Plant, UK, 1973 Saint-Laurent NPP, France, 1980


Buenos Aires Critical Assembly, Argentina, 1983

INCIDENTS
3
SERIOUS INCIDENT
  • External release of radioactivity above authorised 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.
Vandellos NPP, Spain, 1989


2 INCIDENT
  • 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.
 
1 ANOMALY
  • Anomaly beyond the authorised 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").
 
BELOW SCALE/ZERO DEVIATION NO SAFETY SIGNIFICANCE

The doses are expressed in terms of effective dose equivalent (whole body dose). Those criteria where appropriate can also be expressed in terms of corresponding annual effluent discharge limits authorised by National authorities.

 

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