## 5 Proposed Alternative Conceptual Model
### Redox Conditions at the Spent Fuel Surface
The authors propose that this dual effect of H\({}_{2}\) is most likely the result of reactions between reducing species and both oxidants in solution and reactive surface sites, the latter possibly located at grain boundaries. The authors could not identify the nature of the reducing species responsible for these effects on the basis of their data, although they invoke the hydrated electron (e\({}^{-}\text{{}_{aq}}\)) and the \(\bullet\text{H}\) radical. [PERSON] et al. [8] suggests that a combination of alpha-doped UO\({}_{2}\), synthetic and anoxic groundwater, and H\({}_{2}\) is able to: (a) reduce radiolytic oxidants to water, (b) readily reduce traces of external oxidants entering the system, and (c) stabilize the sample surface as stoichiometric UO\({}_{2.00}\). The authors propose that H\({}_{2}\) dissolved in water is adsorbed on the UO\({}_{2}\) surface and acts as electron donor able to reduce oxidizing species formed by radiolysis, and U(V) is potentially formed as a result. [PERSON] [23] proposes the formation of a U(IV)-U(V) site that can then absorb H\({}_{2}\). Thereafter, one \(\bullet\text{H}\) radical is adsorbed and an electron is donated to the surface. Hence, the surface is slightly oxidized but, due to the relatively large amounts of dissolved H\({}_{2}\), a back reaction occurs before the next oxidant brings the U(V) to U(VI). The authors propose that such a process would explain the absence of oxidants and oxidized uranium during the long-term leaching tests mentioned in Section 3, and leads to the overall recombination of oxidizing species in solution back to water. The formation of U(V) and subsequent reduction was recently observed at the surface of a thin UO\({}_{2}\) film exposed to plasma in vacuum at 400 \({}^{\circ}\)C [34; 35]. The authors also mention the formation of a surface-bound \(\bullet\text{OH}\) radical as an intermediate species involved in the oxidation of the UO\({}_{2}\) surface [34; 35; 69]. [PERSON] et al. [70] also suggested that the effect of dissolved H\({}_{2}\) on preventing surface oxidation by alpha radiolysis is apparently a result of the interaction of ionizing radiation with water adsorbed on the surface of actinide oxides, without proposing an exact mechanism but invoking the need of dedicated interfacial radiolysis studies. [PERSON] et al. [70] recognized that, in addition to standard radiolytic processes, energy, charge, or matter can be transferred through the interface, and catalytic or steric effects can alter the decomposition or reactivity of adsorbed molecules. Aqueous phase radiation chemistry can also occur should there be sufficient water near the fuel surface. In that case, homogeneous radiation chemistry will occur with the additional build-up of H\({}_{2}\) due to the confined conditions. Under these circumstances, the Allen cycle [71] will lead to the destruction of oxidizing species produced in the aqueous phase:  
\[\bullet\mathrm{OH+H_{2}\to H_{2}O+\bullet H} \tag{10}\]  
\[\text{h}^{+}+\text{e}^{-}\rightarrow\text{exciton} \tag{2}\]0  
Reactions (10) and (11) will cycle until all the oxidizing species are removed as long as there is excess H\({}_{2}\). Excess H\({}_{2}\) can also be formed by reactive species formed during the interaction of metal oxides with water, as discussed above. The role of reactive species on metal oxide surfaces in accelerating the dissociation of water and producing H\({}_{2}\) in neutral and alkaline solutions has been also proposed in the framework of different studies, such as the development of catalysts for fuel cells [72; 73] or metal corrosion [55; 74; 75; 76]. [PERSON] et al. [74] proposed a model to explain the observed H\({}_{2}\) generation in the presence of copper metal and deoxygenated water involving surface sites available for adsorption of hydroxide intermediates acting as catalysts in the splitting of adsorbed water molecules. According to [PERSON] et al.