Summary of Meeting Paper

The 1996 Annual Meeting of the Society for Risk Analysis-Europe

Communication Nuclear Risks - The International Nuclear Event Scale (INES). R.H. Taylor and S.J. Mortin, Magnox Electric plc., Berkeley Centre, Berkeley, Gloucestershire GLI39PB, UK; B. Thomas and D. Ruatti, International Atomic Energy Agency, Wagramerstrasse 5, P.O. Box 100, A-1400 Vienna, Austria

1. INTRODUCTION

The International Nuclear Event Scale (INES) is a means for promptly communicating, in consistent terms, the safety significance of events reported at nuclear installations. By putting such events into their proper perspective, the intention of the scale is to facilitate a common understanding between the nuclear community, the media and the public.

There are now nearly sixty countries operating the scale - including all of the major users of nuclear power. The original scale, developed for power plant has been extended for use following events at other types of civil nuclear installations. The development of the scale and its use internationally has been facilitated by the International Atomic Energy Agency (IAEA).

2. CRITERIA FOR A SUCCESSFUL SCALE

A number of criteria were established for the development of INES following early work in the UK (1), and prototype scales in France and Japan. These were:

1. The scale should be easy to use and understand. This is an overriding requirement.

2. It should be based on clear technical criteria to promote consistency in use and to ensure that ratings of events on the scale are scientifically defensible.

3. The scale should be capable of being applied quickly to all events.

4. The length of the scale presents potentially conflicting requirements. A "long" scale with fine gradations, provides difficulties of consistent resolution between scale points. On the other hand, there is a need to separate important events (e.g. Chernobyl, TMI) from less significant occurrences. It is also important that not all relatively minor events occupy the lowest point; some discrimination is required to reflect the varying importance of such events.

5. The scale should not conflict with the provisions in emergency plans or with systems of reporting of events established by different national authorities.

6. It should potentially be applicable to reactors of various size and type and to other nuclear installations (e.g. mining, fuel fabrication or reprocessing plant).

3. DEVELOPMENT OF THE SCALE

In developing INES, various attributes of an incident or accident were considered singly or in various combinations to best meet the above criteria. Some were eliminated because they were too subjective, did not relate directly to safety significance or failed to discriminate between minor incidents at the lower end of the scale. A multidimensional scale in the form of a simple matrix was finally agreed as best meeting the above criteria (see fig. 1). Events are classified at seven levels. The lower levels (1-3) are termed incidents and the upper levels (4-7) accidents. Events which have no safety significance are classified as below scale/level 0 and are termed deviations. Events which have no relevance to nuclear or radiological safety (e.g. conventional/industrial accidents) are termed out-of-scale. Three separate criteria are used to classify events. These are off-site impact, on-site impact and defence in-depth degradation. Figure 1 gives an indication of these at each level. An event which possess characteristics represented by more than one column in the matrix is always classified at the highest level achieved in any column. A more comprehensive description of the criteria for each level is given in a User's Manual (2) with detailed guidance on how to rate an event. It also contains many examples as part of the development of "case-law".

Fig. 1. Basic structure of the scale.

4. DEFENCE IN-DEPTH CRITERION

This has been very important in establishing the scientific basis of the scale at its lowest levels. The approach cannot always be directly applied for the wide range of non-reactor facilities to which the scale is now being applied, but the manual provides separate guidance based on the same principles as the reactor guidance and again backed by examples.

The defence in-depth criterion for assessing events in reactors is based on the concept of initiators (events that challenge the safety systems, e.g. loss of off-site power) and safety systems (e.g. diesel generators, to provide a diverse source of power). The rating (0-3) is based on the availability of safety systems, whether or not an initiator occurred and the assumed likelihood of that initiator. The rating is derived from Tables I and II by considering the factors discussed below. The first is applicable for events without an actual initiator (discovered deficiencies in safety system availability) and the second for events where an initiator did occur.

The initiators can have different likely frequencies, i.e., expected (one or several times during the operating life of the plant); possible (not expected, but with an anticipated frequency during the plant lifetime of greater than about 1 %) or unlikely (initiators considered in the design which are even less likely). The availability of a safety function to deal with the initiator can range from full to inadequate:

• Full: All safety systems and components which are provided by the design to cope with the particular initiator to limit its consequences are fully operable.

• Minimum required by operational limits and conditions: The minimum operability of safety systems providing the required safety function specified in the operational limits and conditions for which continued operational at power is permitted, even for a limited time.

• Adequate: A level of operability of safety systems providing sufficient safety function for the initiator being considered.

• Inadequate: The safety systems cannot fulfill the safety function for the initiator under consideration.

The final rating also takes account of other factors that threaten defence in-depth such as common cause failure potential, procedural inadequacies and safety culture deficiencies.

5. EXPERIENCE WITH THE SCALE

Progress in the use and application of the scale has been the subject of constant review. In general, it has been found that the scale is working consistently and well in nearly all countries using it. Media local to many nuclear facilities have become familiar with INES and it is now more widely understood that events rated at the lowest points on the scale are likely to be of minor concern to the public and present no hazard.

Achieving familiarity to a wider audience has been more difficult. Its relevance and use in a national or international context has only rarely been tested. Nevertheless, the IAEA has established a substantial information network between National Officers in each country in order to provide quickly the INES rating of events which might potentially generate international interest. A successful use of the scale occurred in 1992 when a major news item in most European countries suggested a nuclear accident in Russia. A rapid provisional rating by the Russian INES Co-ordinator of level 3 on the scale (later reduced to level 2) was widely quoted in the media and early alarming stories were corrected and the event put into perspective.

There remain several issues in its everyday use. There is a continuing need to reconcile speed of use with the need to justify and explain ratings should they be called into question. To obtain the maximum possible consistency in use and to ensure that there is a firm basis for decisions, users of the scale have asked for increasing guidance and documentation, but this can make the decision making process longer.

Difficulties in use can arise at the boundary between level 0 and level 1 and to a lesser extend between levels 1 and 2. One reason is that operational limits and conditions for plant are specified in different ways. Those, for example, with more exhaustively defined technical specifications are more likely to exceed them from time to time and thus an event at level 1 on the scale will be recorded. This minor variability does not really matter, however, so long as no attempt is made to try to compare performance between countries or between different reactors or reactor types. This is something which the IAEA and its committees has explicitly warned against.

It is possible to debate whether a particular event is at level 0 or level 1 or at level I or level 2, but the crucial point is that the scale is able to distinguish for the public and media between events of true safety significance and those which are minor. This is not to say that minor events do not provide valuable lessons from which much can be learned, but INES is a communication tool and not primarily a means of event analysis and feedback.

REFERENCES

Kelly G.N., Taylor R.H. and Western D.J., 1987, Journal of Radiological Protection, 7 (4).

INES User's Manual, Revised and Extended Edition, 1992 IAEA (Vienna).