Development and Validation of a Model of Uranium Release to Groundwater From Legacy Disposals at the UK Low Level Waste
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Development and Validation of a Model of Uranium Release to Groundwater From Legacy Disposals at the UK Low Level Waste Repository J.S. Small1, C. Lennon1, S. Kwong1 and R.J. Scott2 1 Nexia Solutions Ltd, Warrington, UK 2 LLWR, Holmrook, Cumbria, UK ABSTRACT A previous radiological assessment of the UK Low Level Waste Repository (LLWR) has considered how the prevailing reducing chemical conditions in disposal trenches, may limit uranium release through the extreme low solubility of U(IV) solids. This study considers the additional effects that the physical and chemical nature of the uranium wastes may have on the release of uranium. Fluoride process residues produced by refining of uranium metal comprise the majority of the legacy inventory. Based on historic records and descriptions of the uranium wastes a conceptual model has been developed which bounds the release rate of uranium present as inclusions and dissolved in the solid residues by the dissolution rate of a magnesium fluoride matrix. The model is represented in a 3-dimensional groundwater flow and geochemical model. Initial findings indicate that the model correctly represents the range of fluoride and uranium concentrations that are measured in leachate from the LLWR trenches. Incorporation of this model in future safety assessments, together with a reduction in the derived inventory of uranium, is likely to result in a significant lowering of the peak groundwater dose to acceptable levels, even in the case that the site re-oxidizes. The study builds confidence in the inherent safety features that are provided by the sparingly soluble uranium waste residues and the reducing chemical conditions of the LLWR trenches. INTRODUCTION The UK Low Level Waste Repository (LLWR) located close to the village of Drigg, Cumbria has served as a national repository for LLW since 1959. Until 1988 LLW was backfilled into trenches excavated into glacial clays and covered with an interim cap. The current disposal practice comprises emplacement of compacted waste in steel ISO-freight containers, with void space filled with a cement grout, with the containers stored in an engineered Vault. A post-closure radiological assessment was undertaken in 2002 [1], which highlighted potential doses that may arise from uranium disposals to the trenches. The near-field conceptual model that formed the basis of these calculations [1,2] considered that the release of uranium would be limited by the low solubility of UO2 stable under the prevailing reducing chemical conditions. A biogeochemical reactive transport model of the site [2] indicated that after periods of around 3,000 to 4,000 years the trenches may re-oxidize to the conditions in the surrounding geosphere and uranium may potentially become more mobile. The radiological assessment of the groundwater pathway considering this scenario [1] determined that after 10,000 years Ra-226 and Pb-210 daughters of U-234 would arrive in the biosphere, leading to doses that may exceed the 1e-6 yr-1 risk target. These previous assessments were b
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