Corrosion Studies of Legacy AGR Spent Nuclear Fuel and AGR Simulant Fuels (SIMFuels)

Until November 2018, spent nuclear fuel (SNF) from Advanced Gas-Cooled Reactors (AGRs) in the UK was reprocessed at Thermal Oxide Reprocessing Plant (THORP) at Sellafield. With operations at THORP having now ceased, future plans for un-reprocessed AGR SNF and future discharged AGR SNF, is to dispose of via a national Geological Disposal Facility (GDF). The GDF is expected to start receiving its first waste in the 2040s and may not be open for receipt of spent fuel until ~2075. The current option is storage of AGR SNF in interim storage ponds at Sellafield. These ponds are dosed with NaOH (to pH≈11.4) which acts as a corrosion inhibitor. Current storage periods are typically less than 10 years, although this may extend up to 100 years.<br/><br/>In-reactor or in-pond failed fuels must be safely managed as loss of fuel cladding integrity can lead to potential release of fission products from fuel pellets exposed to coolant or storage pond waters. SNF destined for long term interim wet-storage could face the possibility of pond water ingress through the fuel cladding as a result of compromised cladding over time, resulting in the formation of a corrosion cell at the interface between fuel, cladding and storage solution.<br/><br/>In this study we present the first look at 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. Results are compared with those from corrosion studies of pure UO₂ and AGR SIMFuels specific to UK AGR spent nuclear fuel, conducted at Lancaster University, in order to understand the oxidation behaviour of the UO₂ matrix in failed AGR spent fuel, at low temperatures (T &lt; 200^oC) and under solution conditions relevant to pond storage.<br/><br/>In order to probe the effects of low temperature oxidation/corrosion, a range of electrochemical techniques (chronoamperometry, linear sweep and cyclic voltammetry, and electrochemical impedance spectroscopy) 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 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.<br/><br/>Complementing these real SNF studies, we also present a comparative study of pure UO₂ and AGR SIMFuels coupled with unsensitized 20/25/Nb stainless steel cladding, with experiments carried out under similar conditions as those employed in the above real SNF studies in order to (i) inform the interpretation of results from the latter; and (ii) to validate, in general, the use of spent fuel simulants and non-irradiated fuel cladding as lower activity laboratory analogues. As well as the electrochemical techniques used in the real SNF studies, the SIMFuel samples and the corrosion processes are further analyzed and characterized by use of micro-Raman spectroscopy, X-ray photoelectron spectroscopy and scanning electron microscopy.<br/><br/>These findings will help to underpin options for pre-disposal storage of SNF and predictions of AGR SNF during final disposal and therefore provide satisfaction for the safety case requirements of the UK’s GDF and interim storage plans.