Studies on the solid speciation and remobilisation of plutonium in the northern Irish Sea and southwest coast of Scotland

July 2003

Executive Summary

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 in annual discharges 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 plutonium requires a thorough characterisation of its physico-chemical speciation in the solid phase.

In the first part of this study, an optimised five-step sequential extraction protocol, incorporating the use of sodium citrate to inhibit unwanted resorption, and designed to study the solid partition of plutonium in anoxic sediments, has been thoroughly tested and validated. The methodology employed included certain provisions to preserve the anoxic character of the sediment during analysis.

In the second part, the protocol was used to determine the association of plutonium with different geochemical phases in successive sediment horizons at an intertidal site in the Solway Firth (SW Scotland) and at a subtidal site in the western Irish Sea mud basin. The extensive experimental data produced reveal that plutonium is predominantly bound to exchangeable and acido-soluble geochemical phases. In other words, a significant proportion of the plutonium presently in Irish Sea sediments is in a reversibly bound form associated with more labile geochemical phases. This is in sharp contrast to the findings of previous studies, which may have been skewed by a failure to take account of resorption amongst the remaining geochemical phases in the course of sequential extraction.

Key finding: The study has shown that plutonium bound to sediment in the Irish Sea is potentially more bioavailable than hitherto believed. It follows that this important feature of environmental behaviour should be taken into account in future models designed to predict the long-term radiological impact (and ultimate fate) of plutonium in the Irish Sea system.

Key words: plutonium, solid speciation, remobilisation, Irish Sea sediments

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