Transport and accumulation of actinide elements in the near-shore environment: field and modelling studies



Nuclear facilities in coastal locations often discharge low-level liquid wastes into the sea and the radioisotopes in these discharges are of interest both in assessing possible environmental impacts and as tracers for coastal processes. The distributions of a range of artificial radionuclides, derived from the authorized discharges from British Nuclear Fuels (BNFL) Sellafield, have been determined in the sediments of an intertidal salt marsh in the Esk Estuary, Cumbria, UK. Where published discharge histories exist (for 137Cs, 238Pu, 239,240Pu and 241Am), the sediment core-profile distributions of these radionuclides have been compared with the releases from Sellafield, and consistent values of the accumulation rate (0·226 ± 0·007 g cm−2 yr−1) are obtained. A quantitative model has been developed, describing association of radionuclides with suspended particulate material, which is then accumulated and mixed in an offshore mud patch before resuspension and deposition in the salt marsh. The model has been used to describe radionuclide distributions observed in both the mud patch and the salt marsh, and to identify isotopes for which post-depositional remobilization or solution transport from the discharge point are important. The behaviour of the commonly studied isotopes (137Cs, 238Pu, 239,240Pu and 241Am) is similar to that observed at this and other nearby locations. The activation product isotope 236U is enhanced in these sediments over the natural baseline by four to eight orders of magnitude, and the results suggest that Sellafield-derived uranium is comparably mobile to 137Cs in these sediments although the processes governing the behaviour of these two elements may be different. In situ production of 241Am by decay of its 241Pu parent has generated only 17% of the current sediment inventory of this isotope, insufficient to account for the increase over the last 20–25 years, and suggesting that the input material for these sediments is preferentially enriched in Am relative to Pu during transport from the offshore mud patch. The discharge history of 244Cm, which is unknown, has also been reconstructed from the sediment profile and the model.