Impact of $H_2$ and consecutive $H_2{O}_2$ exposures on the oxidative dissolution of /((U_{1-x}Gd_x)O_2/) pellets under deep repository conditions for spent nuclear fuel

The reactivity of $H_2{O}_2$ towards /((U_{1-x}Gd_x)O_2/) (x = 0.03, 0.045, 0.08) in the presence of $H_2$ was studied experimentally to assess the effect of $H_2$ on oxidative dissolution of Gd-doped $U{O}_2$ under deep repository conditions for spent nuclear fuel. Gd-doped $U{O}_2$ was chosen as a model substance for spent nuclear fuel due to its chemical similarity but low radiotoxicity. While $H_2{O}_2$ is a radiolysis product capable of driving fuel matrix dissolution, $H_2$, which is expected to evolve mainly through the corrosion of cast iron canisters encapsulating the fuel, might limit or even suppress fuel dissolution. Commercial /((U_{1-x}Gd_x)O_2/) pellets were immersed in 10 mm $HC{O}_3^{-}$ solutions and consecutively exposed to 2 mm $H_2{O}_2$ in an autoclave under either 40 bar $H_2$ or 5 bar $N_2$ atmosphere. The results indicate that $H_2$ does not influence the oxidative dissolution of /((U_{1-x}Gd_x)O_2/). Instead, the reactivity of the pellets decreases with increasing $H_2{O}_2$ exposure due to an irreversible alteration of the pellet surfaces resulting in decreased redox reactivity. (© 2020 The Authors. Published by Wiley‐VCH Verlag GmbH and Co. KGaA.)