An investigation of corrosion and leaching of carbide fuels in a GDF setting

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Uranium carbide (UC) is considered an exotic fuel material which has arisen from the UK's civil nuclear reactor test programme. This material has served as a prototype high density fuel at sites like Dounreay in northern Scotland.The Nuclear Decommissioning Authority (NDA) has an inventory of irradiated and non-irradiated uranium carbide as part of its legacy waste exotic fuel materials within its estate and therefore carries the liability for its safe management and ultimately its disposal (for which Radioactive Waste Management (RWM), a subsidiary of the NDA are responsible). Uranium carbide is considered a reactive and potentially pyrophoric material with a reactivity comparable to uranium metal. Geological disposal is internationally recognised as the safest long-term solution for higher activity radioactive wastes and the UK's exotic spent fuels (such as carbide spent fuel) are intended to be managed in this way. The current PhD project will investigate corrosion and leaching behaviour of irradiated and unirradiated carbide spent fuel under conditions analogous to a Geological Disposal Facility (GDF) both (i) pre-closure and (ii) post-closure. Most of the UK's spent fuel inventory is in the form of ceramic uranium dioxide fuels. Oxide provides a stable matrix that is expected to display high chemical stability when contacted by groundwater and, apart from the rapid release of radionuclides at the grain boundaries and in accessible parts of the fuel (the instant release fraction), the rate of release of radionuclides after container failure (the matrix dissolution rate) will be low. Disposal routes for exotic and metallic fuels are yet to be fully determined. It is however expected that carbide and metallic fuels will corrode relatively quickly when accessed by groundwater. This study aim to better underpin the expected behaviour of carbide spent fuel following disposal in a GDF.Experimental Approach:Utilising virgin Uranium carbide fuel material provided by the National Nuclear Laboratory (NNL), Springfields laboratory, the current studentship will use cutting edge materials analysis techniques to provide a nano to micro to millimetre scale observation of carbide corrosion behaviour. Techniques will include X-ray tomography (XRT), high-speed atomic force microscopy, secondary ion mass spectrometry, high-resolution electron microscopy and X-ray diffraction. The techniques are all routinely used and available at the IAC in Bristol. To compliment the materials analysis, leaching studies will utilise solution analysis techniques such as ICP-MS and ICP-OES to determine evolving U concentrations in different GDF-analogous groundwater solutions (oxic and anoxic). In addition, the project will also utilise the unique TRLFS instrument available at the University of Surrey which is being developed for aqueous actinides analysis as part of the main TRANSCEND project.The project will setup a series of enclosed cells experiments, using sealed, water-filled glass housings to hold small uranium carbide 'stick' samples in a fixed position. These special cells will permit periodic measurement of the evolving water chemistry using TRLFS and also the corrosion progression using X-ray tomography. These analysis techniques will enable a detailed study of corrosion and leaching behaviour but without disrupting the experimental system. The lower density of the oxide (10.97 g/cc) which forms from corrosion of the carbide (13.66 g/cc) means that rates of oxidation under different conditions (temperature, dissolved O2 and water chemistry) can be determined by measuring the evolving thickness of the oxide using XRT.