Radionuclide distributions and particle size associations in Irish Sea surface sediments: implications for actinide dispersion

The sediments of the north eastern basin of the Irish Sea contain substantial quantities of contaminant radionuclides, including long-lived actinides, as a consequence of the disposal, from 1952 to the present, of low-level liquid radioactive waste from the Sellafield nuclear fuel reprocessing plant. Onshore transfer of contaminated sediment has resulted in deposition of Sellafield waste radionuclides in intertidal, floodplain and beach environments of this area, with the highest levels of contamination occurring in fine-grained saltmarsh and estuarine deposits. As a result both of reductions in the quantities of radionuclides discharged and of physical dispersion of the contaminated offshore sediment, radionuclide concentrations in sediment deposited in these coastal environments have gradually decreased since the late 1970s. However, the rate of decrease has declined significantly as the contaminant radionuclides have become increasingly dispersed in the offshore sediments and concentrations in the coastal sediments have tended towards those of the offshore sediment. Further reductions in the concentrations of long-lived radionuclides in coastal sediments will therefore depend upon longer-term processes such as re-dissolution or dispersion of the contaminated sediment out of the north eastern basin of the Irish Sea. Results are presented here for a study of radionuclide concentrations in bulk surface sediment and separated size fraction samples of sediments from the Irish Sea and the North Channel. The study reveals that the contaminant radionuclides are dominantly associated with clays and silts in the north eastern basin of the Irish Sea but that at other sampling locations, on a mass balance basis, associations with coarser sediment are often of equal or greater importance. Linear correlations between radionuclide concentrations and comparison of activity ratios with those of the Sellafield discharge suggest that there has been limited dispersion of particles of diameter less than 20 μm from the north eastern basin to other parts of the Irish Sea. The specific activities and the total quantities of radionuclides involved are relatively small, but such a mechanism is nevertheless important in determining the long-term environmental fate of the actinides, which will in turn influence the long-term dose commitment arising from the Sellafield discharge.