THE INFLUENCE OF MIXING TIMESCALES AND RE-DISSOLUTION PROCESSES ON THE DISTRIBUTION OF RADIONUCLIDES IN NORTHEAST IRISH SEA SEDIMENTS

The sediments of the northeast Irish Sea contain substantial inventori es of contaminant radionuclides as a consequence of past discharges of low-level liquid radioactive waste from the Sellafield nuclear fuel r eprocessing plant. Previous work has indicated that onshore transport of this contaminated sediment is the dominant mechanism of supply of S ellafield 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 ho mogenised in the surface offshore sediment, with respect to Pu-238/Pu- 239,Pu-240 and Am-241/Pu-239,Pu-240 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 activi ty ratios of the long-lived radionuclides Pu-238, Pu-239,Pu-240 and Am -241 in the cores exhibited variations that were small relative to tho se 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 a nd with those of sediment deposited in surrounding coastal areas durin g the year after sample collection. In view of the heterogeneous natur e 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 Sell afield waste radionuclides. Co-60 and Cs-134 were detected in the sedi ment to depths ranging from 10 to 18 cm, implying transport to these l evels by mixing on a timescale commensurate with their half-lives. Act ivities of the short-half-life nuclides Ru-106 and Ce-144 decreased wi th depth in the sediment, suggesting a timescale for mixing of the sur face (< 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 t op 10 cm of sediment. The trends in Cs-137/Am-241 activity ratio with depth were consistent with re-dissolution of radiocaesium that had ini tially been taken up in the sediment during the period of high dischar ges from Sellafield. Comparison of the results with Cs-137/Am-241 acti vity ratios for Irish Sea sediment in the late 1970s suggests that up to 77% of the Cs-137 had been re-dissolved from surface (< 10 cm depth ) sediment and some 15% from deeper layers. Finally, it was observed t hat the Cs-137/Am-241 activity ratios for the three cores studied impl ied a K-d of the order of 10(4)-10(5) for radiocaesium in this system. (C) 1998 Elsevier Science Ltd. All rights reserved.