BASIC INTERNATIONAL RADIATION SAFETY APPROACH VIS-À-VIS SAFE DECOMMISIONING

In order to answer the above question, the radiation safety approach formulated in the BSS (on the basis of the ICRP recommendations) need to be analysed. The current international approach to radiation safety divides the possible situations to be regulated by radiation protection standards into two:

— New activities (which are termed ‘practices’), through requirements for design and operational conditions;

— De facto existing situations for which the requirement is for ad hoc pro-tective actions (so-called ‘interventions’).

Conceptually, decommissioning should be planned as part of the practice, and the radiation safety criteria for the termination of a practice should be part of the general radiation safety criteria for practices. However, what happens if decommissioning has not been forecast at the time that the practice was initiated, as is the case for many past practices? Should decommissioning then be treated as an intervention? This dichotomy is at the core of the problem for establishing radiation safety criteria for decommissioning.

The international radiation safety system, be it for practices or for interventions, has to recognize that natural background radiation is exposing everybody in the world. Its levels are very variable: for the majority of people the levels is of the order of 1 mSv/a (the global average is a little higher, 2.4 mSv/a according to UNSCEAR), but with typically high values of around 10 mSv/a that peaks up to around 100 mSv/a and even more in some areas of the world.

4.1. The case of ‘practices’

If the introduction of a practiceis justified, taking account of its expected benefits and detriments, it is accepted that the operation of the practice will entail an increase in the background dose — i.e. an additional dose will be

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caused by the practice, however small such a dose might be, which will be added to the existing background dose as a result of that practice (Fig. 3). This fact is accepted in international standards and in any national regulation, otherwise it would imply a prohibition of practices. The international standards impose a limit of 1 mSv/a to the additional dose from all controlled practices, i.e. they accept a duplication of the minimum background dose as a result of the operation of all practices.

Since the limitation could cover the additional annual doses arising from several practices, each containing several sources, dose constraints per source are also required. At this time a dose constraint of 0.3 mSv/a is being recommended by the ICRP, with a constraint of 0.1 mSv/a for prolonged components. Below the constraint the standards require a process of optimization of protection,i.e. reduction of the remaining doses to levels judged

‘as low as reasonably achievable’ (or ALARA) under the prevailing circumstances, which will indicate the final dose restriction imposed for a particular source. A level of exemption has been agreed internationally: it is in the order of 0.01 mSv/a, which should be underlined because it could be important for termination of practices (see Fig. 4).

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FIG. 3. How the system works for practices.

4.2. The case of ‘interventions’

For interventions, the situation is of reduction rather than addition of doses. If an extant dose exists in a de facto existing situation, e.g. an area containing radioactive residues, the first radiation safety question is whether such an extant dose should be reduced, i.e. whether it is justified to intervene with protective actions to reduce the dose. If the answer to this first question is positive, then the second question is by how much such a dose should be reduced, i.e. how to optimize the protective actions required by the justified intervention. It is obvious that some residual dose, however small, will remain after the intervention (see Fig. 5).

This means, therefore, that there will be a ‘new extant (background) dose’

after the reduction of the extant dose by the intervention. It follows that during the process of intervention it is nonsense to apply dose limits: the objective is to reduce doses as much as reasonably achievable, not limit them.

The international criteria for interventions in situations with prolonged extant doses, which are recommended by the ICRP, are based on the level of such extant doses. Basically, they can be formulated as follows (Fig. 6):

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FIG. 4. Limiting the individual dose from practices.

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FIG. 5. How the system works for interventions.

FIG. 6. Dose criteria for justifying intervention with protective actions.

— When doses are very high (e.g. approaching 100 mSv/a), intervention is almost always justifiable, i.e. intervention ‘must’ be performed. The qual-ifier ‘almost’ recognizes the fact that there are a few places in the world where the doses are that high and nothing is being done against them.

— Intervention may probably still be justified when the extant dose approaches values of around 10 mSv/a.

— When the values of the extant dose are very near the average background doses (i.e. in the region of a few mSv per year) intervention is not likely to be justifiable.

These criteria compare well with natural background levels around the world, which were referred to heretofore.

4.3. Summary: How the present system works

In summary, the present system of radiation protection works on the assumption that there is an extant background dose with an average of 2.4 mSv/a, which in some areas of the world may rise to a typical high of around 10 mSv/a, or a very high value of around 100 mSv/a and even more.

Intervention is rarely justifiable at the low end, may be justifiable in the middle and is always justifiable at the upper end. Above this extant background dose value there are additional doses that may be introduced by justifiable practices, which are restricted with a dose limit of 1 mSv/a, a source constraint of 0.3–0.1 mSv/a and reduced further with a process of optimization of radiation protection; there is an exemption value of 0.01 mSv/a (Fig. 7).