The influence of mixing timescales and re-dissolution processes on the distribution of radionuclides in Northeast Irish Sea sediments

Abstract

The sediments of the northeast Irish Sea contain substantial inventories of contaminant radionuclides as a consequence of past discharges of low-level liquid radioactive waste from the Sellafield nuclear fuel reprocessing plant. Previous work has indicated that onshore transport of this contaminated sediment is the dominant mechanism of supply of Sellafield waste radionuclides to surrounding intertidal and floodplain areas, but an inherent, and previously unsubstantiated, assumption in this work was that past discharges from Sellafield are effectively homogenised in the surface offshore sediment, with respect to 238Pu 239,240Pu and 241Am 239,240Pu activity ratios, as the result of mixing on a timescale of a few years. Results are presented here for a study of three cores, of lengths ranging from 10 to 38 cm, collected from the deposit of contaminated offshore sediment in 1992. The activity ratios of the longlived radionuclides 238Pu, 239,240Pu and 241Am in the cores exhibited variations that were small relative to those in the Sellafield discharge, indicative of a high degree of mixing of the sediment. Moreover, the sediment activity ratios were found to be compatible with those of the Sellafield time- integrated discharge and with those of sediment deposited in surrounding coastal areas during the year after sample collection. In view of the heterogeneous nature of the sediments in the northeast Irish Sea, caution must obviously be applied in any generalisation based upon the analysis of only three cores, but for the locations sampled, the results are consistent with the surface sediment acting as a well mixed source of long-lived Sellafield waste radionuclides. 60Co and 134Cs were detected in the sediment to depths ranging from 10 to 18cm, implying transport to these levels by mixing on a timescale commensurate with their half-lives. Activities of the short-half-life nuclides 106Ru and 144Ce decreased with depth in the sediment, suggesting a timescale for mixing of the surface (< 13 cm depth) sediment of the order of 1 year. On the basis of the distribution of these short-half-life radionuclides, biodiffusion coefficients of the order of 10 −7 cm 2 s −1 were derived for the top 10 cm of sediment. The trends in 137Cs 241Am activity ratio with depth were consistent with re-dissolution of radiocaesium that had initially been taken up in the sediment during the period of high discharges from Sellafield. Comparison of the results with 137Cs 241Am activity ratios for Irish Sea sediment in the late 1970s suggests that up to 77% of the 137Cs had been re-dissolved from surface (< 10 cm depth) sediment and some 15% from deeper layers. Finally, it was observed that the 137Cs 241Am activity ratios for the three cores studied implied a K d of the order of 10 4–10 5 for radiocaesium in this system.