Accumulation of technetium-99 in the Irish Sea?

An assessment has been carried out to determine the impact of continued (99)Tc discharges into the Irish Sea from the nuclear fuels reprocessing plant at Sellafield. Samples of surface and bottom seawater and sediment have been collected from the Irish Sea and analysed for (99)Tc. The information has been used, together with supporting data, to determine the effect of summer stratification upon the seawater concentrations and to evaluate whether sediments provide a sink for (99)Tc. Hydrographic data provide clear evidence of thermal stratification of waters above the muddy sediment in the western Irish Sea. Surface water contained higher (99)Tc concentrations than bottom water, and concentrations were inversely related to water salinity. This inverse relationship was not observed in the eastern Irish Sea close to Sellafield. (99)Tc activities in surficial sediments were greatest (>20 Bq/kg) at sites closest to the Cumbrian coastline. Activity, from equivalent sampling sites, remained similar between surveys carried out in 1995 and 1998. The muted response of the seabed sediments, to fluctuations in the Sellafield discharges, compared with the water column is to be expected given that they reflect the integrated radionuclide discharge history.

An estimate of the inventory of technetium-99 in the sub-tidal sediments of the Irish Sea.

Published results from earlier studies have provided indications that measurable quantities of technetium-99 ((99)Tc) have accumulated in the sub-tidal sediments of the Irish Sea. This is due to the enhanced discharges from the Sellafield nuclear reprocessing plant in Cumbria, UK (between 1994 and 2004). Depth distributions of (99)Tc concentrations in sub-tidal sediments have been determined from a limited number of Irish Sea sites, following the collection of deep sediment cores (up to 2 m in depth), sampled in two research cruise surveys in 2005 and 2006. Vertical concentration profiles of (99)Tc from a range of substrates in the Irish Sea are presented here and these have been used to produce an estimate of the total inventory of (99)Tc residing in the sub-tidal sediments of the Irish Sea. Significant variation was observed between (99)Tc concentrations in the sediment samples, as well as in the shape of individual depth profiles. As anticipated, concentrations tended to be greater on fine-grained (muddy) substrates and showed a general decrease with distance from Sellafield. Vertical concentration profiles of (137)Cs, and (137)Cs data from published work, have also been considered to evaluate the use of the relatively few (99)Tc core data (upon which to determine the (99)Tc inventory). The inventories of (99)Tc and (137)Cs are estimated to have been of the order of 30 and 455 terabecquerels (TBq), respectively, or ∼2% of the total cumulative Sellafield discharge for each of the two radionuclides. The residence half-time of (137)Cs in the sub-tidal sediments of the Irish Sea is estimated to be in the order of ∼16 years. Therefore, as the Kd values for (99)Tc and (137)Cs are similar, this also provides an indicative value to predict future losses of (99)Tc from the sediment reservoir.