Concentration and depuration of some radionuclides present in a chronically exposed population of mussels (Mytilus edulis)

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Abstract
Factors were described which affected the concentration (p Ci/g dry wt) and loss of 241Am, 239 + 240Pu, 238Pu, 144Ce, 137Cs, 134Cs, 106Ru, 95Zr and 95Nb in an exposed population of mussels M. edulis from Ravenglass on the Esk estuary, Cumbria, UK, which received radioeffluents from the British Nuclear Fuels Ltd. (BNFL) plant at Sellafield, .apprx. 10 km to the north. Tidal position and mussel body size had a negligible influence on the concentration of 241 Am, 137 Cs and 106Ru in the total soft tissue, but variation in soft tissue weight throughout the year had a considerable influence on the apparent concentration and depuration times of these radionuclides. Apart from the clearance (biological half-life [t1/2], 1 to 3 h) of sediment-associated activity from the digestive tract, the depuration rate prolifes followed a single component clearance curve with a biological t1/2 in excess of 200 d [day] for 241Am, 239 + 240Pu, 238Pu and 144Ce and 40 d for 137 Cs. The clearance of 106Ru was more complex and consisted of a 3 component depuration profile with biological half-lives of 6 h, 12 and 260 d. The depuration profiles were for chronically ingested isotopes under natural conditions; acute exposure would most likely result in different profiles, especially those derived from laboratory spiking experiments. Isotope ratio data supported the hypothesis that the main route of entry into the mussel for the majority of the radionuclides was from the water. Differences in the biological half-lives observed between 239 + 240Pu and 238Pu and 241Am could have been related to their different physico-chemical forms rather than a biological mechanism differentiating between these isotopes; as there were no suitable data available for the composition (chemical or physical) of the BNFL effluent it was not possible to determine whether these differences reflected procedures employed during fuel reprocessing or sediment-water interactions after discharge. This paucity of information also made it difficult to determine the degree to which the Ravenglass mussels reflected recent discharges from BNFL.