The Corrosion of UO2-Based Spent Nuclear Fuel Under Storage and Disposal Conditions—A Comparison of Real Irradiated Fuel and SIMFUEL Studies

Historically, UO2-based spent nuclear fuel (SNF) in the UK was reprocessed at Sellafield’s Thermal Oxide Reprocessing Plant (THORP). THORP closed in 2018 and current UK policy is that SNF will be send to a GDF due to open ~2075. Until then fuel will be kept in storage ponds at Sellafield.<br/>Internationally, the vast majority of UO2-based SNF is from LWRs and a significant amount of research has been carried out to support both the wet storage and direct disposal concepts in terms of the physical and aqueous durability of cladding and irradiated UO2. However, in the UK, the vast proportion of SNF is from indigenous Advanced Gas-cooled Reactors (AGRs). AGRs, whilst also using UO2-based fuel, employ CO2 as coolant and are graphite moderated. Further, the fuel assembly cladding is comprised of 20/25/Nb steel (20% Cr, 20% Ni) rather than zircalloy as is the case in PWRs. Consequently, AGR fuel has unique characteristics that need to be evaluated in order to satisfy safety case requirements for both extended pond storage and geological disposal.<br/>In the UK, the very high radiation fields arising from real SNF prohibits their study in all but a few specialised “hot cells” operated by Sellafield or NNL, a resource that is necessarily constrained. One way to obviate this problem is to work on simulated SNF (SIMFUELs<br/>Thus, we have been studying a series of novel SIMFUELs designed to replicate SNF discharged from a UK AGR at a range of simulated burnups. AGR SNF destined for long term wet-storage could face pond water ingress through the fuel cladding as a result of damage to that cladding. Similarly, in a GDF, at some point after repository closure, it is expected that the canisters within which the SNF is sealed will fail, allowing the ingress of groundwater. In either scenario, the pond/groundwater may come into contact with the UO2 fuel pellets initiating their corrosion and dissolution.<br/>Consequently, our studies have focussed on understanding both the materials properties of these SIMFUELs and their corrosion behaviour as a function of electrolyte compositions and conditions relevant to pond storage and geological disposal. This presentation will present the key findings from those studies, with an emphasis on those conducted electrolyte systems designed to simulate storage pond waters and granitic and evaporite groundwaters.<br/>Features to be presented include: the role of each of the main classes of additives to the SIMFUEL (the trivalent lanthanides, the so-called grey phases, the epsilon-particles) play in determining the behaviour of the fuel matrix and the effect of galvanic coupling of the SIMFUEL to the stainless steel cladding on their corrosion behaviours (inc. the sacrificial protection afforded to the fuel in some groundwaters).<br/>This SIMFUEL data is compared with recently obtained novel data gathered from electrochemical corrosion studies of UK legacy AGR spent nuclear fuel and conducted in the “hot cell” facilities at the UK National Nuclear Laboratory (NNL) Windscale Facility. In these studies of real fuel, a range of electrochemical techniques have been deployed on a cross-sectional segment of real irradiated AGR spent nuclear fuel, under electrolyte conditions of different solution pHs and chloride concentrations in order to mimic certain aspects of pond storage conditions. This data provides both (i) the first ever insight to the electrochemical corrosion behaviour of a SNF sample consisting of irradiated uranium dioxide coupled directly to similarly-irradiated 20/25/Nb stainless steel and (ii) confirmation that the non-active SIMFUEL studies provide an excellent surrogate for the study of real irradiated fuel.