Homogeneity and Realism in Critical Group Assessments for Routine Radiological Discharges

The 'critical group' concept provides a robust means of demonstrating compliance with radiological dose limits set for routine effluent discharges arising from operational nuclear plants. Calculating the average dose to members of the critical group based on reasonable 'maximising assumptions' offers a defensible degree of conservatism in demonstrating compliance with the dose limits and, in principle, the identification of a 'relatively homogeneous' group, representative of those individuals in the population expected to receive the highest radiation dose, appears to offer no difficulties. Over the past twenty years, since publication of ICRP 26 guidance, however, there have been a number of changes to dose limits and interpretation of critical groups. In practice this has resulted in seeking more realistic dose estimates. Where a number of different habits are relevant to exposure (e.g. consumption of marine foods, fishing, occupancy of intertidal areas and components from terrestrial pathways) the requirements for homogeneity and realism may conflict. Radiological assessments based on critical groups, and associated environmental monitoring programmes, need to be sensitive to changes in individual pathways without necessarily switching the focus of presentation in a way which may appear haphazard to members of the public. A balanced approach is thus required in defining group characteristics, and presentation of ranges on estimated dose may be appropriate. These issues are considered in the context of assessment of doses to marine critical groups in West Cumbria. (author)