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 (⁹⁹Tc) 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 ⁹⁹Tc 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 ⁹⁹Tc 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 ⁹⁹Tc residing in the sub-tidal sediments of the Irish Sea. Significant variation was observed between ⁹⁹Tc 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 ¹³⁷Cs, and ¹³⁷Cs data from published work, have also been considered to evaluate the use of the relatively few ⁹⁹Tc core data (upon which to determine the ⁹⁹Tc inventory). The inventories of ⁹⁹Tc and ¹³⁷Cs 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 ¹³⁷Cs in the sub-tidal sediments of the Irish Sea is estimated to be in the order of ∼16 years. Therefore, as the K_d values for ⁹⁹Tc and ¹³⁷Cs are similar, this also provides an indicative value to predict future losses of ⁹⁹Tc from the sediment reservoir