## 5 Proposed Alternative Conceptual Model
### Processes Occurring at the Metal Oxides-Water Interface  
The radiolysis of water adsorbed at the surface of metal oxides, including UO\({}_{2}\), shows high (\(\sim\)10-fold) radiolytic yields of molecular H\({}_{2}\) in adsorbed water compared to that formed by radiolysis of pure water, whereas the yields of O\({}_{2}\) were at least an order of magnitude lower for water adsorbed on UO\({}_{2}\) or ZrO\({}_{2}\) surfaces [42; 43; 49; 50]. The possible mechanism for excess H\({}_{2}\) production in the radiolysis of adsorbed water proposed by [PERSON] and [PERSON] [42] (and references therein) involve dissociative electron attachment of low-energy electrons, recombination of electron-hole pairs, and exciton reactions. The negligible production of O\({}_{2}\) in these systems may be explained by the decomposition of water bound at the interface, resulting in oxygen species attached to or near the oxide surface. In 1971, [PERSON] observed that water associated with oxide surfaces is usually dissociated to form OH groups in the form of hydroxylated metal oxide surfaces (\(\equiv\)M-OH) [51]. Additional water will be hydrogen-bound to this first chemisorbed layer in the form of physisorbed water layers. The fate of O\({}_{2}\) in these systems is unclear. Pulse radiolysis studies suggest that only the reducing species leave the surface to the bulk water [52]. XPS studies of irradiated materials report non-stoichiometric oxygen within the bulk material [53]. The oxygen may be filling up vacancies, but it is most likely breaking up the crystal structure, a process well known in radiation-induced corrosion studies [54]. As mentioned above, self-irradiation of UO\({}_{2}\) can cause the formation of oxygen vacancies (O\({}_{v}\)). Oxygen vacancies (O\({}_{v}\)) are also known to play an important role in the surface chemistry of metal oxides such as CeO\({}_{2}\) and AmO\({}_{2}\) (which are fluorite-typestructures often used as UO\({}_{2}\) analogues) and TiO\({}_{2}\)[55; 56]. Hydroxylated species (\(\equiv\)M-OH) and hydride species (\(\equiv\)M-H) bonded to the surface can act as oxidizing or reducing species [55; 56]. It is also known that oxygen vacancies have the tendency to spontaneously ionize, and thus automatically liberate electron polarons in the lattice and reduce the surrounding metal atoms [57]. First-principles studies of the reaction of water on the crystallographic surfaces of UO\({}_{2}\) and PuO\({}_{2}\) (and ThO\({}_{2}\)) help in explaining the excess formation of electrons and H\({}_{2}\). [PERSON] et al. [58] propose that H\({}_{2}\) is formed from water dissociation. Water dissociation may undergo two pathways in the presence of surface oxygen vacancy on the reduced UO\({}_{2}\)(111) surface. One path is characterized by direct combination of two hydrogen atoms of one water molecule, and the other is characterized by dissociation of the first hydrogen atom and its combination with a neighboring surface hydrogen atom. [PERSON] et al. (2019) [59] propose that excess electrons are formed from radiation-induced oxygen vacancies. The excess electrons lead to the exothermic splitting of H\({}_{2}\)O and formation of molecular H\({}_{2}\) on UO\({}_{2}\) surfaces. This effect depends on the nature of the oxide; for example, for PuO\({}_{2}\) the formation of H\({}_{2}\) from interfacial water is thermodynamically unfavorable. Calculations have reported that energies associated with adsorbed and dissociated water are highly dependent on the specific UO\({}_{2}\) surface [58; 60]. Water adsorbed on a pristine stoichiometric UO\({}_{2}\) surface is experimentally found to be a reversible process at room temperature [61]. The evolution of H\({}_{2}\) is not observed in temperature programmed desorption studies of these surfaces, indicating that the energy binding the water to the UO\({}_{2}\) surface is relatively low [62]. However, water is dissociatively adsorbed on UO\({}_{2}\) surfaces with U in a lower oxidation state or from highly defected surfaces.