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 Kd values for 99Tc and 137Cs are similar, this also provides an indicative value to predict future losses of 99Tc from the sediment reservoir.