The significance of distribution coefficients in the remobilisation of actinides from intertidal sediments of the Ravenglass Estuary

Distribution coefficients (Ksub(d)) were determined for desorption of actinides from Ravenglass Estuary intertidal sediment into 'inactive' seawater from the North Sea. Values of 1.1 x 106 for 239+240Pu and 2.0 x 106 from 241Am were obtained. Further experiments indicated the desorption-sorption equilibrium reaction for plutonium is an oxidation-reduction process, the sediment itself able to reduce Pu V+VI to Pu III+IV, whereas for americium the formation of a soluble carbonate complex is thought to occur. Using seawater diluted with river water, Ksub(d) values were found to decrease by approx. two orders of magnitude at low values of salinity, characteristic of low tide conditions and in the upper estuary. Tidal balance experiments indicated the Ravenglass Estuary currently acts as a radionuclide source to the Irish Sea, actinides being lost from the estuary in suspended sediment. Distribution coefficients for 239+240Pu measured during three tidal cycles were approx. = 106 at low water but decreasing rapidly on the flood. The effect was more pronounced in calm conditions and did not follow the relationship between desorption ksub(d) and salinity observed in the laboratory experiments. (author)