Impact of H2 and Consecutive H2O2 Exposures on the Oxidative Dissolution of (U1–xGdx)O2 Pellets Under Deep Repository Conditions for Spent Nuclear Fuel

The reactivity of H2O2 towards (U1–xGdx)O2 (x = 0.03, 0.045, 0.08) in the presence of H2 was studied experimentally to assess the effect of H2 on oxidative dissolution of Gd‐doped UO2 under deep repository conditions for spent nuclear fuel. Gd‐doped UO2 was chosen as a model substance for spent nuclear fuel due to its chemical similarity but low radiotoxicity. While H2O2 is a radiolysis product capable of driving fuel matrix dissolution, H2, which is expected to evolve mainly through the corrosion of cast iron canisters encapsulating the fuel, might limit or even suppress fuel dissolution. Commercial (U1–xGdx)O2 pellets were immersed in 10 mm HCO3– solutions and consecutively exposed to 2 mm H2O2 in an autoclave under either 40 bar H2 or 5 bar N2 atmosphere. The results indicate that H2 does not influence the oxidative dissolution of (U1–xGdx)O2. Instead, the reactivity of the pellets decreases with increasing H2O2 exposure due to an irreversible alteration of the pellet surfaces resulting in decreased redox reactivity. With increasing H2O2 exposure the dissolution yield of Gd‐doped UO2 decreases. This indicates a change in the redox reactivity of Gd‐doped UO2 upon the exposure to H2O2. An inhibiting effect of H2 on the oxidative dissolution of Gd‐doped UO2 could not be observed.