Factors are described which affect the concentration (p Ci g-' dry wt) and loss of ^Am, 239 + 240Pur 2 3 8~u r 144Ce, 13?Cs, '^Cs, lo6Ru, ^Zr and ^Nb in an exposed population of mussels Mytilus edulis L. from Ravenglass on the Esk estuary, Cumbria, UK which receives radioeffluents from the British Nuclear Fuels Ltd. (BNFL) plant at Sellafield, some 10 km to the north. Tidal position and mussel body size have a negligible influence on the concentration of 241Am, l3?Cs and ^Ru in the total soft tissue, but variation in soft tissue weight throughout the year has a considerable influence on the apparent concentration and depuration times of these radionuclides. Apart from the clearance (ti,, biol, 1 to 3 h) of sediment-associated activity from the digestive tract, the depuration rate profiles follow a single component clearance curve with a biological half-life in excess of 200 d for ^'Am, 239 + ^OPU,
23%and l4Ve, and of 40 d for 13'Cs. The clearance of Io6Ru is more complex and consists of a 3 component depuration profile with biological half-lives of 6 h, 12 d and 260 d. The depuration profiles presented in this work are for chronically ingested isotopes under natural conditions; acute exposure will most likely result in different profiles, especially those derived from laboratory spiking experiments. Isotope ratio data support the hypothesis that the main route of entry into the mussel for the majority of the radionuclides studied is from the water. Differences in the biological half-lives observed between 239 + ^OPu and both and 241Am could be related to their different physico-chemical forms rather than a biological mechanism differentiating between these isotopes; as there are no suitable data available for the composition (chemical or physical) of the BNFL effluent it is not possible to determine whether these differences reflect procedures employed during fuel reprocessing or sediment-water interactions after discharge. This paucity of information also makes it difficult to determine the degree to which the Ravenglass mussels reflect recent discharges from BNFL.
Rates of uptake of 2 4 1~m , I3'Cs and ' 0 6~u by both the soft tissue and the shell of transplanted Mytilus edulis L. were determined; soft tissue data are compared to theoretically derived values based on the biological half-life and the steady state concentration of these isotopes. The rates of loss of 2 4 1~m , U 9 + 2 4 0~~, 2 3 8~~, I3'Cs and I o 6~u by M. edulis under conditions of stress, manifest as extended periods of shell closure as a consequence of high ambient concentrations of copper and zinc and extended periods of aerial exposure, were examined. Both types of stress did not significantly alter the rate of loss of the Pu isotopes, 13'Cs or I o 6~u from the flesh of the mussel but the 2 4 1~m concentration increased significantly over the depuration period of February to June -a probable consequence of the remobilisation of 2 4 1~m associated with the inner nacreous layer of the shell.
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