Corrosion Behaviour of Agr Simfuels
Authors: Rauff-Nisthar Nadya , Boxall Colin , Hambley David , Hiezl Zoltan , Padovani Cristiano , Wilbraham Richard 
Published: February 2020

In the UK, the majority of spent nuclear fuel (SNF) is from indigenous Advanced Gas-cooled Reactors (AGRs). AGRs, whilst using UO2-based fuel, employ CO2 as coolant and are graphite moderated. Further, the fuel assembly cladding is comprised of 20/25/Nb stainless steel rather than zircalloy as is the case in Pressurised Water Reactors (PWRs). Consequently, AGR fuel has unique characteristics that need to be evaluated before disposal in an expected anoxic UK geological repository.

Pure UO2 pellets and, for the first time, AGR SIMFUEL pellets simulating 25 (pellet interior) and 43 GWd/tU burn-up (rim structure) have been fabricated at the UK National Nuclear Laboratory. Using combined electrochemical and Raman spectroscopic studies, we have investigated the effect of the SIMFUEL dopants on the UO2 crystal matrix. We have also studied the effect of H2O2 on the SIMFUEL surface as a simulant for the effect of a radiolysis on intruding groundwater in the near field.

Raman microscopy reveals an increase in the lattice damage (500 – 700 cm-1) with increasing burn-up, as the UO2 cubic fluorite lattice structure becomes more distressed and moves towards a tetragonal structure. Cyclic voltammetric studies of the AGR SIMFUELs reveal them to more susceptible to electrochemical oxidation then PWR SIMFUELs; in particular the peak associated with the in-grain UO2 to UO2+x oxidation (onset -0.2 V vs SCE,  Ep = +0.14 V) is ~3X larger for AGR SIMFUELs than PWR SIMFUELs of similar simulated burnup. Importantly, analogous studies on PWR SIMFUELs have shown that UO2+x locations act as preferential corrosion sites.

The open circuit potential (EOC) for UO2 and AGR SIMFUELs show a logarithmic dependence on [H2O2] in modified simplified groundwater(10 mmol dm-3 NaCl, 2 mmol dm-3 NaHCO3, pH 6.8). In contrast, the EOC of PWR SIMFUELs of ~0.1 V vs SCE in aqueous alkaline solution is near independent of [H2O2], an observation attributed to surface-catalysed H2O2 decomposition by non-stoichiometric U(IV)/U(V) sites. This suggests that, in contrast to the non-complexing alkaline conditions, carbonate complex-ation of U(VI) in groundwater simulant removes the H2O2 decomposition sites from the UO2 surface so resulting in a larger net [H2O2] and thus oxidative stress being induced at the SIMFUEL surface.

At [H2O2] = 10 mmol dm-3, close to that encountered in an a dominated near field, H2O2 generates an oxidative stress of 0.05 V vs SCE on 43 GWd/tU AGR SIMFUEL in simplified modified groundwater. This corresponds to the near top of the voltammetric wave for the in-grain UO2 to UO2+x process, suggesting that near field peroxide may be capable of inducing a substantial population of anodically active UO2+x sites on AGR SNF and consequent localised site corrosion at the spent fuel surface.

Extended duration Raman studies on the oxidation of UO2 on 25 GWd/tU SIMFUELs in 1 mmol dm-3 H2O2 (EOC  = +0.19 V vs SCE) in O2-free deionised water reveal that, over a 1 week period, there is an increase over in the intensity of the peak at 540 cm-1, associated with the formation of defect sites in the UO2 fluorite lattice. Shorter term studies on undoped UO2 samples in 0.01 to 1 mmol dm-3 H2O2 solutions also reveal the in-growth of a peak at ~610 cm-1, a feature that has been attributed to O sublattice distortions due to the generation of UO2+x locations. This again indicates of the potential for near field concentration H2O2 to induce an increased susceptibility to corrosion for AGR SIMFUEL.

The ultimate product of H2O2 induced corrosion of either UO2 or AGR SIMFUELs would be expected to be U(VI). The extended duration Raman studies show that a peak appears at 820 cm-1 after 5 hours exposure to 1 mmol dm-3 H2O2. This grows in intensity up to 50 hours exposure and has been assigned to metastudtite (UO2(O2) 2H2O), a uranium peroxide secondary phase formed at the SIMFUEL surface as a result of the reaction between H2O2 and UO2
2+ – the latter necessarily generated as a result of above-expected H2O2–driven corrosion of the AGR SIMFUEL UO2 matrix.

Peroxide aside, open circuit potential measurements on 43 GWd/tU simulated burnup AGR SIMFUELs samples in intimate contact with 20/25/Nb stainless steel and in modified simplified groundwater electrolyte find that they exhibit a mixed potential of ~-0.12 V vs SCE. This corresponds to nearly a quarter of the way up the oxidation wave for in-grain UO2 to UO2+x process in the associated voltammogram – implying that UO2/steel coupling can induce UO2 corrosion even in the absence of a peroxide generated oxidative stress.