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 Sea^1–5, 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 sediment^6–12. Sellafield discharges^13,14 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 magnitude^16,17, is largely concentrated in restricted areas of fine sediment off the Cumbrian coast close to Sellafield^3,4,18 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 likely^19,20, 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.