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

Time-series data on plutonium, americium and Cs-137 concentrations in surfi cial 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 radionu clides 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 Pb-210-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 north eastern Irish Sea to the western Irish Sea. The proposed interpretation has been supported by the analysis of transuranium concentrations and radionuc lide ratios along two east-west transects from Sellafield, which has highli ghted the importance of sediment remobilisation processes in the dispersion of particle-reactive radionuclides post-input. These processes have been i ncorporated in a low-resolution compartmental model of the Irish Sea and us ed to predict future Pu-239,Pu-240 concentration trends in the western Iris h Sea mud basin, Model predictions for Pu-239,Pu-240 in the surface sedimen t and seawater compartments in this zone, validated by comparison with inde pendent 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 Cs-137, th, decline is much more rapid , the corresponding availability times being at least an order of magnitude shorter. (C) 1999 Elsevier Science Ltd. All rights reserved.