General description of the scale:
The International Nuclear Event Scale (INES) is a means for promptly communicating to the public in consistent terms the safety significance of events reported 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 in 1989 by the International Atomic Energy Agency (IAEA) and the Nuclear Energy Agency (NEA) of the Organization for Economic co-operation and Development. 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.

The Scale was initially applied for a trial period to classify events at nuclear power plants and then extended and adapted to enable it to be applied to any event associated with radioactive material and/or radiation and to any event occurring during transport of radioactive material. It is now operating successfully in over 60 countries.

The INES Information Service, the communication network built up on request receives from and disseminates to the INES National Officers of 60 Member States, Event Rating Forms that provide authoritative information related to nuclear events. Event Rating Forms are circulated when events are significant for:

The communication process has therefore led each participating country to set up a structure which ensures that all events are promptly rated using the INES rating procedure to facilitate communication whenever they have to be reported outside.

Events are classified on the Scale at 7 levels; the upper levels (4-7) are termed accidents and the lower levels (1-3) incidents. Events which have no safety significance are classified below scale at level 0 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. Each level is defined in detail within the INES User's 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 critical group 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 damage to the reactor core or radiological barriers).

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.

Events which do not reach the threshold of any of the criteria are rated below scale at level 0.

The back page of this leaflet gives typical descriptions of events at each level together with examples of the classification of nuclear events which have occurred in the past at nuclear installations.

Using the Scale

  • The detailed rating procedures are provided in the INES User's Manual. This leaflet should not be used as the basis for rating events as it only provides examples of events at each level, rather than actual definitions.
  • Although the Scale is designed for prompt use following an event, there will be occasions when a longer time-scale is required to understand and rate the consequences of an event. In these rare circumstances, a provisional rating will be given with confirmation at a later date. It is also possible that as a result of further information, an event may require reclassification.
  • The Scale does not replace the criteria already adopted nationally and internationally for the technical analysis and reporting of events to Safety Authorities. Neither does it form a part of the formal emergency arrangements that exist in each country to deal with radiological accidents.
  • Although the same Scale is used for all installations, it is physically impossible at some types of installation for events to occur which involve the release to the environment of considerable quantities of radioactive material. For these installations, the upper levels of the Scale would not be applicable. These include research reactors, unirradiated nuclear fuel treatment facilities, and waste storage sites.
  • The Scale does not classify industrial accidents or other events which are not related to nuclear or radiological operations. Such events are termed "out of scale". For example, although events associated with a turbine or generator can affect safety related equipment, faults affecting only the availability of a turbine or generator would be classified as out of scale. Similarly, events such as fires are to be considered out of scale when they do not involve any possible radiological hazard and do not affect the safety layers.
  • The Scale is not appropriate as the basis for selecting events for feedback of operational experience, as important lessons can often be learnt from events of relatively minor significance.It is not appropriate to use the Scale to compare safety performance among countries. Each country has different arrangements for reporting minor events to the public, and it is difficult to ensure precise international consistency in rating events at the boundary between level 0 and level 1. The statistically small number of such events, with variability from year to year, makes it difficult to provide meaningful international comparisons.
  • Although broadly comparable, nuclear and radiological safety criteria and the terminology used to describe them vary form country to country. The INES has been designed to take account of this fact.

Examples of rated nuclear events

Basic Structure of the Scale(Criteria given in matrix are broad indicators only)Detailed definitions are provided in the INES User's 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 COUNTER-MEASURES

5 ACCIDENT WITH OFF-SITE RISK

LIMITED RELEASE: LIKELY TO REQUIRE PARTIAL IMPLEMENTATION OF PLANNED COUNTER-MEASURES

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/OVER EXPOSURE OF A WORKER

INCIDENTS WITH SIGNIFICANT FAILURES IN SAFETY PROVISIONS

1 ANOMALY

ANOMALY BEYOND THE AUTHORIZED OPERATING REGIME

0 DEVIATION

NO

SAFETY

SIGNIFICANCE

The International Nuclear Event Scale For prompt communication of safety significance

LEVEL / DESCRIPTOR

NATURE OF THE EVENTS

EXAMPLES

7 ACCIDENTS / 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 of 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 in 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 installation. 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, NPP, USA,

1979

4 ACCIDENT WITHOUT SIGNIFICANT OFF-SITE RISK

External release of radioactivity resulting in a dose to the critical group of the order of a few millisieverts *. 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 installation. Such an accident might include damage 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 resulting 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

3 INCIDENT / SERIOUS INCIDENT

External release of radioactivity resulting in a dose to the critical group 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. These include events where the actual failures would be rated at level 1 but which reveal significant additional organizational inadequacies or safety culture deficiencies. 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 authorized regime but with significant defence-in-depth remaining. This may be due to equipment failure, human error or procedural inadequacies and may occur in any area covered by the scale, e.g. plant operation, transport of radioactive material, fuel handling, waste storage. Examples include: breaches of technical specifications or transport regulations, incidents without direct safety consequences that reveal inadequacies in the organizational system or safety culture, minor defects in pipework beyond the expectations of the surveillance programme.

DEVIATIONS 0 BELOW SCALE

Deviations where operational limits and conditions are not exceeded and which are properly managed in accordance with adequate procedures. Examples include: a single random failure in a redundant system discovered during periodic inspections or tests, a planned reactor trip proceeding normally, spurious initiation of protection systems without significant consequences, leakages within the operational limits, minor spreads of contamination within controlled areas without wider implications for safety culture.

NO SAFETY SIGNIFICANCE

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