Geochemical fractionation of plutonium in anoxic Irish Sea sediments using an optimised sequential extraction protocol

J.A. Lucey, A Gouzy*, D. Boust, L. Vintro, L. Bowden, P. Mitchell (* étudiant en thèse à l'IRSN)

14th International Conference on Radionuclide Metrology and its Applications,

02/06-06/06, Dublin Irlande.

For over four decades, sediments from the NE Irish Sea have acted as a temporary sink for radionuclides discharged from the Sellafield spent nuclear fuel reprocessing plant sited in Cumbria, England. With the steady reduction of discharge levels since, the mid-1970s, the relative importance of remobilisation processes has become increasingly apparent. This is the case for plutonium, where the contribution of historic discharges associated with sediment is now the main source of this element to the overlying water column. Evaluation of the mobility and bioavailability of a given radionuclide requires a thorough characterisation of its physico-chemical speciation in the solid phase. In the context, sequential extraction protocols have been used extensively to identify the primary geochemical host phases involved in radionuclide uptake and to give insight into possible physico-chemical species .

In our study, an optimised five-step sequential extraction protocol, incorporating the use of sodium citrate as a deterrent to resorption effects, has been used to assess fractionation of plutonium in subtidal and intertidal anoxic sediments collected in the Irish Sea. Techniques used to preserve the anoxic conditions conditions of the sediments prior to analysis and the maintenance of an oxygen free environment during the extraction discussed. Detailed data on the solid partitioning of plutonium from cores retrieved from an intertidal site in the Esk Estuary, a saltmarsh in the Solway Firth and also subtidal sites in the Irish Sea are presented. The results reveal that plutonium is predominantly bound to the exchangeable and the acido-soluble fractions, indicating that a significant proportion of the plutonium presently in Irish Sea  sediments is associated with the more labile phases.