The system of radiation protection

The system of radiation protection is embodied in a set of radiation protection requirements contained in the Agency’s Basic Safety Standards [2]. In turn, the Basic Safety Standards are based primarily on the recommendations of the ICRP.

3.4.1. Principles

For proposed and continuing practices, such as transport, the system of protection recommended by ICRP [1] is based on the general principles given below:

(1) Practices should produce sufficient benefit to offset the radiation harm that they may cause (justification):

(2) In relation to any particular source within a practice, the magnitude of the individual doses, the number of people exposed, and the likelihood of exposures where they are not certain to be received, should be kept as low as reasonably achievable (ALARA), economic and social factors being taken into account. This procedure should be constrained by restrictions on the doses to individuals (dose constraints), or the risks to individuals in the case of potential exposures (risk constraints), so as to limit the disparity likely to result from the inherent economic and social judgements (the optimization of protection); and

(3) Individual exposure from all sources susceptible to control are subject to dose limits and some control of risk from potential exposures (dose and risk limits).

3.4.2. Justification

The Basic Safety Standards states that, “No practice or source within a practice should be authorized unless the practice produces sufficient benefit to the exposed individuals or to society to offset the radiation harm that it might cause; that is, unless the practice is justified, taking into account social, economic and other relevant factors.”

In the very few cases where decisions on the justification of transport operations are called for, the principle should be regarded in a general way. It would be sensible to state that all the merits and harm associated with the practice and the possible alternatives under consideration should be taken into account in reaching the decision.

3.4.3. Dose limits

The Basic Safety Standards states that, “The normal exposure of individuals shall be restricted so that neither the total effective dose nor the total equivalent dose to relevant organs or tissues, caused by the possible combination of exposures from authorized practices, exceeds any relevant dose limit specified in Schedule II, except in special circumstances provided for in Appendix I. Dose limits shall not apply to medical exposures from authorized practices.”

3.4.3.1. Individual dose limits

It is important to recognize that dose limits are the third part of the ICRP recommendations [1]. They are set so that any continued exposure just above the dose limits would result in additional risks that could be reasonably described as "unacceptable" in normal circumstances.

There are basically two requirements in setting the dose limits. The first is to keep doses below the threshold level for deterministic effects and the second is to keep the risk of stochastic effects at a tolerable level. The stochastic effects occur at considerably lower doses and are the basis for dose limitations. The dose limits recommended by ICRP [1] are

summarized in Table 3.5.

TABLE 3.5. DOSE LIMITS

Occupational Public

Effective dose 20 mSv/y averaged over 5 consecutive years^a

aAdditional restrictions apply to the occupational exposure of pregnant women.

bIn special circumstances, an effective dose of 5 mSv could be allowed in a single year, provided that the average over 5 years does not exceed 1 mSv per year.

The choice of a dose limit is not, and cannot be, a matter of science and scientific judgement alone. The final decision is essentially a political and economic one that needs to be made as part of the national regulatory process. The ICRP [1] and BSS-recommended limits [2] represent a professional judgement based on a complex and sophisticated multi-attribute analysis

3.4.3.2. Potential exposures and risk limits

Not all exposures occur as forecast. There may be accidental departures from planned operating procedures, or equipment may fail. Such events can be foreseen and their probability of occurrence estimated, but they cannot be predicted in detail. The ICRP recommends that the individual and collective harm resulting from an exposure that should not occur shall be included in the system of radiation protection. Ideally, dose limits should be supplemented by risk limits that take account of both the probability of incurring a dose and the harmful effects of that dose if it were to be received.

3.4.4. Optimization

ICRP states that "optimization of protection", "keeping all exposures as low as reasonably achievable, economic and social factors being taken into account", and "ALARA"

are identical concepts within the ICRP system.

A wide range of techniques is available to optimize radiation protection. Some of these techniques are drawn from operational research, some from economics, and some from engineering. The techniques available include procedures based on cost-benefit analysis, which are discussed in detail in the ICRP report on optimization of protection. It is important to recognize that other techniques, some quantitative, others qualitative, may also be used in the optimization of radiation protection. IAEA-TECDOC-374 [5] contains a discussion of, and guidance on, the optimization of radiation protection in the transport of radioactive material. Clearly, in practice, and particularly in day to day operations, there will be little opportunity to undertake complex quantitative calculations to determine what is optimal, and professional judgements may need to be made on a qualitative and, sometimes, intuitive basis.

There are nevertheless a number of situations where the formal techniques such as

cost-benefit analysis can provide a valuable aid to decision-making in radiation protection.

Their application is most likely in circumstances where the decisions are complex and the expenditure potentially large.

3.4.5. Dose constraints

The Basic Safety Standards states, “Except for medical exposures, the optimization of the protection and safety measures associated with any particular source within a practice shall be subject to dose constraints that:

(a) do not exceed either the appropriate values established or agreed to by the Regulatory Authority for such a source or values which can cause the dose limits to be exceeded;

and

(b) ensure, for any source (including radioactive waste management facilities) that can release radioactive substances to the environment that the cumulative effects of each annual release from the source be restricted so that the effective dose in any year to any member of the public, including people distant from the source and people of future generations, is unlikely to exceed any relevant dose limit, taking into account cumulative releases and the exposures expected to be delivered by all other relevant sources and practices under control.”

The concept of a dose constraint sets an upper limit on the exposure of an individual to a single source. The dose constraint does not replace optimization; rather it sets a ceiling on the levels of individual dose to be considered in the optimization of protection for that source.

Its purpose is three-fold:

(1) To ensure that the dose limit is not exceeded when exposures to all sources are summed, (2) To further constrain individual inequalities when benefits and detriments are unevenly

distributed among individuals, and

(3) To enable regulatory authorities to establish a dose constraint for a particular source based on a knowledge of good practice.