The interfacial radiation chemistry of UO2 is of key importance in the development of models to predict the corrosion rate of spent nuclear fuel in contact with groundwater. Here, the oxidative dissolution of UO2 induced by radiolytically produced H2O2 is of particular importance. The difficulty of fitting experimental data to simple first-order kinetics suggests that additional factors need to be considered when describing the surface reaction between H2O2 and UO2. It has been known for some time that UO22+ forms stable uranyl peroxo-carbonato complexes in water containing H2O2 and HCO3-/CO32-, yet this concept has largely been overlooked in studies where the oxidative dissolution of UO2 is considered. In this work, we show that uranyl peroxocarbonato complexes display little to no reactivity toward the solid UO2 surface in 10 mM bicarbonate solution (pH 8-10). The rate of peroxide consumption and UO22+ dissolution will thus depend on the UO22+ concentration and becomes limited by the free H2O2 fraction. The rate of peroxide consumption and the subsequent UO22+ dissolution can be accurately predicted based on the firstorder kinetics with respect to free H2O2, taking the initial H2O2 surface coverage into account.

H2O2 produced from water radiolysis is expected to play a significant role in radiation induced oxidative dissolution of spent nuclear fuel under the anoxic conditions of a deep geological repository if the safety-barriers fail and ground water reaches the fuel. It was recently found that the coordination chemistry between U(vi), HCO32− and H2O2 can significantly suppress H2O2 induced dissolution of UO2 in 10 mM bicarbonate. This was attributed to the much lower reactivity of the U(vi)O22+-coordinated O22− as compared to free H2O2. We have extended the study to lower bicarbonate concentrations and explored the impact of ionic strength to elucidate the rationale for the low reactivity of complexed H2O2. The experimental results clearly show that dissolution of U(vi) becomes suppressed at [HCO3−] < 10 mM. Furthermore, we found that the reactivity of the peroxide in solutions containing U(vi) becomes increasingly more suppressed at lower carbonate concentration. The suppression is not influenced by the ionic strength, which implies that the low reactivity of O22− in ternary uranyl-peroxo-carbonato complexes is not caused by electrostatic repulsion between the negatively charged complex and the negatively charged UO2-surface as we previously hypothesized. Instead, the suppressed reactivity is suggested to be attributed to inherently higher stability of the peroxide functionality as a ligand to UO22+ compared to as free H2O2.

In the event of multi-barrier failure in a deep geological repository for spent nuclear fuel, dissolution of the fuel in groundwater is primarily driven by radiolytic oxidants, mainly H₂O₂. Dissolution of oxidized UO2 (i.e., UO₂²⁺) is enabled by the presence of HCO₃^-/CO₃^2- which forms soluble uranyl-carbonato complexes. In solutions containing H₂O₂, UO₂²⁺ and HCO₃^-/CO₃^2-, ternary complexes can also be formed. In this work we have studied the impact of UO₂²⁺ on the stability of peroxide in bicarbonate solutions under irradiation. To elucidate the contributions by the primary and secondary radiolysis products CO₃^·-, H^·, e_aq^- and O₂^·-, three different systems were studied; N₂O-purged (CO₃^·-), N₂-purged (CO₃^·-, H^· and e_aq^- combined contribution) and O₂-purged (CO₃^·- and O₂^·- combined contribution). In the N₂O-purged solutions, the presence of 300 μM UO₂²⁺ approximately doubled the rate of peroxide loss (in solutions initially containing 200 μM H₂O₂). In N₂-purged solutions UO₂²⁺ did not affect the initial rate of peroxide consumption but was observed to suppress peroxide loss at lower [H₂O₂]. In the O₂ purged solutions, the H₂O₂-steady-state concentration was approximately 5 times higher in the absence of uranyl compared to the solution containing 300μM UO₂²⁺. Irradiation of bicarbonate solutions, initially free from peroxide with varied [UO₂²⁺], revealed a complex [UO₂²⁺] dependence on the peroxide stability. An initial buildup of H₂O₂ occurs at a rate independent of [UO₂²⁺], whilst a UO₂²⁺-catalyzed peroxide consumption occurs after some buildup of H₂O₂. At [UO₂²⁺] > 100μM a decrease in H₂O₂-steady-state concentration occurs. At [UO₂²⁺] < 100μM, buildup of H₂O₂ continues at a suppressed rate and eventually reaches the same steady state concentration as in the UO₂²⁺-free solution. These observations are attributed to a UO₂²⁺-catalyzed Haber-Weiss type cycle where O₂^·- selectively reduces UO₂²⁺-coordinated peroxide. Accumulation of UO₂²⁺ beyond 100μM is therefore expected to suppress oxidative dissolution of UO₂ by suppressing the buildup of H₂O₂.

Radioactive nuclide release rates upon groundwater intrusion in a failed repository are largely governed by the rate at which the fuel matrix is dissolved. Several authors have reported mechanistic interpretations related to the carbonate facilitated dissolution of oxidized UO₂. However, these accounts have studied the dissolution in conjunction with the oxidation process. Unlike the oxidation process, the dissolution process has not been studied separately and therefore remains unresolved.  In this work we have investigated the carbonate concentration- and temperature dependence on the rate of dissolution of pre-oxidized UO₂ in the absence of oxidant. The apparent activation energy of the dissolution was found to increase with the carbonate concentration within the concentration range (5 – 15) mM and temperature range (283 – 333) K. This is attributed to an adsorption isotherm, where the negative contribution of the adsorption equilibrium causes a lower apparent activation energy in the low carbonate concentration region. Carbonate concentration dependencies were found to vary within the range 0.5 – 1.5 at temperatures ≤ 295 K, and between 1 – 1.6 at temperatures ≥ 303 K. The dependence of the surface concentration of oxidized product on the rate of dissolution showed three distinct regions; (I) a fast initial release of a minor fraction, (II) a constant release rate and (III) a gradual decrease in the rate of dissolution as the oxidized product nears depletion. Interestingly, most of the release (> 70 percent) is released at a constant rate. This indicates that the carbonate-available surface remains relatively unaffected by the amount of oxidized product remaining in the solid throughout most of the dissolution.

Understanding the oxidative dissolution of spent nuclear fuel (SNF) is crucial for predicting the fate of radiotoxic elements in geological repositories. The oxidation of UO_₂, the main component of SNF, is largely driven by H₂O₂, which decomposes on UO_₂ to generate OH^•. Molecular hydrogen (H_₂) can counter this oxidation through reduction catalyzed by ε-particles (noble metal inclusions) and potentially by direct H-abstraction between H₂ and OH^•. The interaction between UO₂, OH^• and H_₂ was studied in the absence and presence of palladium (Pd) doping. The UO_₂ thin films were synthesized and exposed to water plasma under ultra-high vacuum conditions to introduce OH^•, while purging the surface with molecular H2. X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) were used to monitor changes in oxidation state and the near-surface electron configuration. Our results show that without Pd doping, H₂ does not significantly inhibit UO_₂ oxidation. However, Pd-doped UO_₂ films resist oxidation under water plasma at 400 °C. 

Groundwater intrusion upon multi-barrier failure in a deep geological repository is a plausible scenario that could lead to radioactive nuclide release into the environment. Due to the reducing conditions of groundwater at the depth of the repository, coupled with the low solubility of UO₂, the rate of degradation of the fuel matrix is largely governed by radiolytically induced redox process that can both promote- (oxidation) and inhibit (reduction) – dissolution of oxidized UO_2+x. The presence of metal inclusions in the fuel matrix such as Pd has been shown to effectively inhibit oxidative dissolution in the presence of dissolved H₂. In our previous work we demonstrated that the inclusion of a 15 percent Pd:U ratio kept UO₂ thin films fully reduced under otherwise  highly oxidizing H₂O-plasma at 400^oC. In this work we have explored the influence of temperature on the efficiency of the Pd-catalyzed, H₂ induced reduction of UO₂ thin films under H₂O-plasma. Unlike exposures at higher temperatures, the reduction occurs only partially at room temperature, from U(VI)O₃ to U(V) (likely UO₂OH), with no indication of secondary reduction to U(IV)O₂. Additionally, we found that full oxidation of the Pd-catalyst (into PdO) leads to irreversible loss of catalytic function, likely through disintegration of  Pd clusters and Pd/U/O alloy formation.