Mechanisms for northwards dispersal of Sellafield waste

Abstract

Almost all of the plutonium and americium discharged as liquid effluent from Sellafield is retained in the sediments of the north east Irish Sea1–5, whereas the waste radiocaesium shows relatively conservative behaviour in seawater and is mostly dispersed into more distant waters with only a small fraction being retained in Irish Sea sediment6–12. Sellafield discharges13,14 considered in the light of radionuclide geochemistry and radioactive decay and grow-in15 imply that, by the end of 1984, the sediments of the north-east Irish Sea contained about 6 x 102 TBq each of 239,240Pu and 241Am and of the order of 103TBq of 137Cs. This contamination, of globally significant magnitude16,17, is largely concentrated in restricted areas of fine sediment off the Cumbrian coast close to Sellafield3,4,18 so it is important to identify processes which could lead to the dispersal of these nuclides to areas of potential human contact (such as intertidal sediments) over the long time scales appropriate to the half lives of 241AM Am (458 yr), 239Pu (2.44 x 104 yr) and 240Pu (6.58 x 103 yr). Because no significant return of these actinides to solution in seawater is likely19,20, most concern must be centred upon physical dispersal processes affecting the contaminated sediment. These are ill-defined, although Hunt21 contends that actinides incorporated in intertidal sediments near Sellafield are subject to mixing with previously discharged waste and to dispersal, with removal from the surface sediments within one or two decades. Here we present the results of a study of radionuclide concentrations in surface (<0.5cm depth) intertidal sediments collected during 1984–85 which provide the basis for an estimate of the extent of northwards dispersal of the contaminated sediment and an assessment of the importance of this process relative to transport of radionuclides in solution.