Trends in plutonium, americium and radiocaesium accumulation and long-term bioavailability in the western Irish Sea mud basin

Time-series data on plutonium, americium and 137Cs concentrations in surficial sediments and seawater, gathered at selected locations in the western Irish Sea mud basin in the period 1988–97, are examined and modelled in an attempt to reconstruct the deposition history of Sellafield-sourced radionuclides in the basin and estimate representative mean availability times for these nuclides in this zone. Predictions using a semi-empirical modelling approach have been confirmed by the analysis of 210Pb-dated sediment cores retrieved from the basin, and interpreted in terms of the processes likely to control the transport of particle-reactive radionuclides from the northeastern Irish Sea to the western Irish Sea. The proposed interpretation has been supported by the analysis of transuranium concentrations and radionuclide ratios along two east–west transects from Sellafield, which has highlighted the importance of sediment remobilisation processes in the dispersion of particle-reactive radionuclides post-input. These processes have been incorporated in a low-resolution compartmental model of the Irish Sea and used to predict future 239,240Pu concentration trends in the western Irish Sea mud basin. Model predictions for 239,240Pu in the surface sediment and seawater compartments in this zone, validated by comparison with independent data sets, indicate that concentrations have already peaked and are beginning to decline slowly with mean availability times of ca. 100 years in both compartments. In the case of 137Cs, the decline is much more rapid, the corresponding availability times being at least an order of magnitude shorter.