Safety_Series_041_1975 - gnssn - International Atomic Energy ...

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This publication is no longer valid Please see http://www.ns-iaea.org/standards/ 52 A PPEN D IX II (b) those where activity is not detectable, or at a very low concentration in critical materials but is easily measurable in indicator m aterials; and (c) those where activity is not detectable in any materials. The latter situation may still require a limited monitoring programme for what might be termed public relations purposes. The second category of situation can be observed through the medium of an indicator m aterial which is usually chosen for high reconcentration of potentially critical radionuclides. The first category requires continued study of the exposed population, because a given concentration of the critical nuclide in the critical material will not result in the same degree of radiation exposure for each member of that population. The eventual aim, therefore, is to isolate that group in the population whose habits, location or age lead to their receiving the highest doses. These people then become the critical group and the control limits are applied to the average of this group. In practice, great care is required in selecting this group and,in designing and modifying the monitoring programme to reflect accurately its radiation exposure and further surveys of the habits of the critical population are required from time to time. The specific activity approach This basic approach, descriptions of which appear in several publications, stems from a knowledge of the perm issible human organ or body burden for a particular radionuclide as laid down by ICRP (Table VI). This is then expressed as a specific activity in relation to the concentration of the stable nuclide of the same element in that organ or the body, e .g . p C i/g of element x. This specific activity becomes the limiting activity concentration for the water, on the assumption that if this concentration is not exceeded in the water, then it cannot be exceeded at any point in the food chain or in the critical organ in man. The rate of introduction of activity is set so that this ratio in the water is not exceeded. This approach is very attractive because it dispenses with the concentration factor data (required by the critical pathway system when carrying out the initial assessment) and is supposedly applicable regardless of the eating habits of the individual concerned, thus dispensing with the need for habits surveys. It is, of course, only applicable to food-chain evaluations, and in the application of
This publication is no longer valid Please see http://www.ns-iaea.org/standards/ APPENDIX II 53 TABLE VI. SPECIFIC ACTIVITY APPROACH TO DISPOSAL ASSESSMENT FOR AQUEOUS RADIOACTIVE WASTE ICRP m axim u m p erm issible organ (body) burden o f rad ionuclid e I Organ (body) c o n ten t o f stable nuclid e M axim u m perm issible sp e c ific a c tiv ity in seaw ater & C o n cen tratio n o f stable e le m e n t in seaw ater M axim um p erm issible co n cen tra tio n o f rad ionuclide in seaw ater & Equilibriu m c o n cen tra tio n from u n it rate o f discharge M axim u m p erm issible rate o f discharge this approach to coastal sites care must be taken to ensure that external dose rates on nearby beaches due to the adsorption of radionuclides on sediments is not excessive. The approach has limitations with respect to elements where data on human stable nuclide concentrations are not available. Nor can it be applied to those cases where the critical organ for the radionuclide or radionuclide mixture is the gastro-intestinal tract. This method is m ore easily applied to sea disposal than to disposal into fresh water since the chem ical com position of seawater is m ore uniform. The use of the stable nuclide distribution of an element as an analogue for the radionuclide distribution involves several simplifying assumptions, most of which will lead to conservative rates of radionuclide introduction. However, the most important assumption is that of the relative biological availability of the stable and radioactive nuclides. The greatest variability of stable nuclide concentrations of trace elements is likely to be encountered in coastal waters, where the majority of waste disposal operations are undertaken. Furtherm ore, the limiting situation posed by most m ajor discharges of radioactivity relates to areas close to the point of release, where it is unlikely that the freshly . introduced radionuclide will rapidly assume the distribution of its
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