The TRANSCEND Consortium - Management of Spent Nuclear Fuels - 22097

The management of spent nuclear fuels is a major concern for the UK owing to the cessation of reprocessing operations at Sellafield and the complex inventory arising from Magnox, AGR, PWR and prototype reactors. Retrieval and relocation operations for legacy fuels are imminent and therefore, any models that enhance our understanding of fuel evolution will help mitigate the risks associated with fuel storage, retrieval and disposal. Research into the hydraulic behaviour of perforated pins and likely ingress into the fuel annulus through pellet cracks has focussed on microcosm tests where simulants of spent mixed oxide fuels have been fabricated to provide an insight into the behaviour of mixed actinide matrices. Voltammetric studies comparing their electrochemical activity to pure uranium dioxide pellets have revealed substantial differences, indicative of likely dissolution behaviour. Similar concerns surround the integrity of steel cladding during AGR fuel storage owing to stress corrosion cracking; consequently, there is an urgent need for better understanding of its behaviour during drying. Laboratory rigs have been built for micromechanical, drop evaporation tests on stainless steel plate samples and novel small-scale punch-tests for in situ investigations. A mathematical model has been developed to determine the length and width of cracks in the fuel cladding whereas parallel research into thermal sensitisation of austenitic steels has helped elucidate the relationship between grain boundaries and chromium depletion. TRANSCEND has allowed further development of a suite of advanced laser spectroscopic techniques for real-time monitoring of spent fuel corrosion. Time-resolved laser fluorescence (TRLFS) and multi-laser Raman spectroscopy allow detailed structural characterisation of alteration products, to which laser-induced breakdown spectroscopy (LIBS) has been added to provide compositional information. A representative selection of uranyl oxide, peroxide, phosphate and silicate minerals has been analysed as the first stage in constructing a comprehensive spectral library for nuclear applications. The aim is to deploy at least some of these techniques, which are currently laboratory-based, in situ and at full-scale via miniaturised, fibre-optic probes. The work packages described above form the basis of a predictive tool for spent fuel corrosion. To date, a one-dimensional, fuel dissolution model has been developed, together with a fully functioning alpha dose rate calculator, which includes crack and spherical geometries. The latter can be used to assess the radiolytic generation of H₂, H₂O₂, OH^-, etc. in a range of fuel - water exposure scenarios. The highly integrated research programme under TRANSCEND Theme 3 represents a significant opportunity for enhanced monitoring of the UK's spent nuclear fuel inventory during storage. Nevertheless, substantial uncertainties remain concerning the fate of spent fuel, particularly during disposal; the reasons behind this are discussed. (authors)