Development and validation of a model of uranium release to groundwater from legacy disposals at the UK Low Level Waste Repository

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. (authors)