An iterative approach to evaluation

The step-wise iterative procedure recommended in this report to assess critical group doses is illustrated in Fig. 21. This approach has the advantage that it represents an efficient use of assessment resources in that, if applied correctly, it allows resources to be targeted at those aspects of the assessment that give the highest doses.

To apply this method a suitable reference level of dose is needed to indicate when a more accurate dose assessment, and the use of a more complex model, is required. It is recommended that this reference level be set to take account of both the relevant dose limiting criterion and the uncertainty associated with the dose assessment model, as discussed in Section 2.1.1.

8.1.1. Initial assessment steps

The first step in this iterative process is to assess doses on the basis of the no dilution model. If doses calculated using this approach are less than the relevant dose criterion (e.g. dose constraint), no further model complexity is needed, since this approach is very conservative and actual doses received would be expected to be much lower than those predicted by this method. If doses exceed the dose criterion a greater level of accuracy in the model predictions is warranted and the generic environmental model is recommended.

The second step is to apply the generic environmental model. If the critical group doses predicted using this model are less than a reference level (e.g. one tenth of a dose constraint specified by a Regulatory Authority), no further model complexity is needed. However, if doses exceed the reference level a re-evaluation of the assumed input data is recommended, as indicated in Fig. 21.

8.1.2. Re-evaluation of input data

The objective of this stage in the process is to review the input data applied in making the generic assessment discussed in the previous section, in order to determine whether they are unduly pessimistic in relation to the conditions at the site considered. Examples of the data that may be considered are discussed below.

8.1.2.1. Estimated discharge rate and conditions

In this process the first step is to re-evaluate the estimated discharge rate and the conditions of discharge to confirm that they are not overestimates. If the discharge rate has been overestimated the generic dose calculations may be repeated using a revised estimate.

FIG. 21. An iterative approach for assessing critical group doses giving examples of factors that may be considered when checking the relevance of generic assumptions to a site.

Apply the no dilution model

No

Apply the site specific model with advice from a suitable expert

(beyond the scope of this report) Perform modified generic assessment of generic assumptions for

the most important nuclides and pathways

For example the discharge rate, discharge conditions, wind speed or water flow

rate

For example if inhalation is important it would be reasonable to consider the effect of soil migration on the doses

received The assumptions regarding

the location of the critical group or food production

8.1.2.2. Exposure conditions

If, after review of the discharge rate and discharge conditions, the revised calculated doses are still greater than the reference level, it is recommended that the exposure conditions, assumed as part of the generic methodology, should be re-evaluated and revised to reflect more closely the conditions actually prevailing at the site. This re-evaluation process should begin with those conditions that apply to the radionuclides and exposure pathways that dominate the dose estimate. Any changes in the exposure assumptions that lead to a revised estimate of the hypothetical critical group dose should be accompanied by a detailed justification of these modifications.

The variables in the generic methodology that are likely to be most amenable to site specific re-evaluation and modification are shown below.

(a) The location of members of the hypothetical critical group and of food production;

(b) Dietary habits and residence times of members of the hypothetical critical group;

(c) Annual average flow rate of a surface water body;

(d) Annual average wind speed;

(e) Specific exposure pathways;

(f) K_dvalues for the specific site and the specific chemical forms of radionuclides at the site;

(g) If external doses are important, it may be necessary to consider migration of radionuclides in soil.

After the initial estimates of dose are completed, the dominant radionuclides and exposure pathways should be identified and evaluated. This evaluation may reveal that some of the pathways assumed in the generic models are absent at the site.

Before these pathways are eliminated from a revised generic calculation, care must be taken to account for the presence of other food types not explicitly considered in the generic methodology. The generic food types included in this methodology are intended to serve as surrogates for any food type that might actually be present. Thus the generic meat pathway is intended to be sufficiently robust to account for the harvesting of wild game. The generic fish pathway is also intended to account for the consumption of other aquatic biota that are not specifically identified in the model, including waterfowl. The combined use of the generic pathways of milk, meat and vegetables (including the default values for consumption rates) is intended to account for hypothetical critical group consumers of forest mushrooms.

Some of the generic pathways included will, however, always be present at a given site. For example, if it is possible that people occupy the lands surrounding an atmospheric discharge location, inhalation will always have to be taken into account.

Similarly, external exposure to radionuclides deposited on the ground will always have to be considered if an atmospheric discharge contains γemitting particulates. Intakes through local food products will also be of general relevance, but the distances between the farm or garden where local foods are produced and the discharge location vary.

8.1.3. Final revised generic dose calculations

Once the adjustments for site specific conditions are made, the generic calculations should be repeated. If the dose estimates still exceed the reference level, it is recommended that the following be considered.

— Reducing the amount discharged,

— Modifying the conditions of the discharge,

— Consulting professionals in environmental radiological assessments to perform a detailed site specific assessment.

8.2. REALISTIC DOSE ASSESSMENTS IN CONSULTATION WITH