To assess the long-term leaching behaviour of UO2, the main constituent of spent nuclear fuel, the oxidative dissolution of UO2 pellets was studied at high H2O2 exposures ranging from 0.33 mol m-2 to 1.36 mol m-2. The experiments were performed in aqueous media containing 10 mM HCO3- where the pellets were exposed to H2O2 three consecutive times. The results indicate that the dissolution yield (amount of dissolved uranium per consumed H2O2) at high H2O2 exposures is significantly lower compared to previous studies of both pellets and powders and decreases for each H2O2 addition for a given pellet. This implies a change in redox reactivity, which is attributed to irreversible alteration of the pellet surface. Surface characterization after the exposure to H2O2, by SEM, XRD and Raman spectroscopy shows, that the surface of all pellets is significantly oxidized.