## 5 Proposed Alternative Conceptual Model
### Redox Conditions at the Spent Fuel Surface  
As observed in some of the experimental studies discussed in Section 3, we know that radiation plays an important role in explaining the role of H\({}_{2}\) in suppressing UO\({}_{2}\) dissolution. Although there is consensus on the fact that radiolytic species are involved in the processes occurring at the UO\({}_{2}\) surfaces, it is still to be demonstrated which species cause oxidation of U(IV) to U(VI) (possibly involving the intermediate species U(V)) and whether there are species capable of reversing the effect of oxidation so that the net balance of oxidizing and reducing species maintains the spent fuel surface in a reduced state, i.e., no oxidizing dissolution occurs in reducing conditions. We present below two possible mechanisms producing an excess of reducing species (either \(\bullet\)H, e\({}^{-}\)aq or H\({}_{2}\)) that may be responsible for maintaining the spent fuel surface in a reduced state, hence precluding the oxidative dissolution of UO\({}_{2}\).The following mechanism is based on the work of [PERSON] et al. [68] for water adsorbed on TiO\({}_{2}\). Radiation first induces the formation of electron (e\({}^{-}\)) -hole (h\({}^{+}\)) pairs in UO\({}_{2}\), as shown in Equation (1):  
\[\text{UO}_{2}\ \text{(intrinsic gamma, beta and alpha irradiation)}\rightarrow\text{h}^{+}+\text{e}^{-} \tag{1}\]  
where h\({}^{+}\) is an electronic defect vacancy (see Section 5.2). The charge carriers can combine to give an exciton (hole-electron coupled state on the surface)--Equation (2):  
\[\text{h}^{+}+\text{e}^{-}\rightarrow\text{exciton} \tag{2}\]  
It has been suggested that excitons in hydroxylated metals react with the surface water groups \(\equiv\)U-OH, as shown in Equation (3). The possible formation of hydroxylated groups (\(\equiv\)U-OH) also on the surface of spent nuclear fuel via the formation of oxygen vacancies is discussed in Section 5.3. Lastly, \(\bullet\)H atoms combine to form H\({}_{2}\)--Equation (4):  
\[\text{exciton}+\equiv\text{U-OH (+H${}_{2}$O)}\rightarrow\equiv\text{U-O} \bullet+\bullet\text{H} \tag{3}\]  
\[\bullet\text{H}+\bullet\text{H}\rightarrow\text{H}_{2} \tag{4}\]An additional surface reaction producing H\({}_{2}\) can be a hydrogen atom abstraction from a hydroxylated metal surface:  
\[\bullet\text{H}+\equiv\text{U-OH}\rightarrow\equiv\text{U-O}\bullet+\text{H}_{2} \tag{5}\]  
There is not normally enough adsorbed water to form the hydrated electron, but with nearby bulk water, Equation (3) can take place in the aqueous phase along with the following combination reactions:  
\[\text{e}^{-}\text{{}_{aq}}+\text{e}^{-}\text{{}_{aq}}+2\text{H}_{2}\text{O} \rightarrow\text{H}_{2}+2\text{OH}^{-} \tag{6}\]  
\[\text{e}^{-}\text{{}_{aq}}+\bullet\text{H}+\text{H}_{2}\text{O} \rightarrow\text{H}_{2}+\bullet\text{OH} \tag{7}\]  
Another pathway for \(\bullet\text{H}\) production may be the electron attachment to \(\equiv\)U-OH groups (Equation (8)) followed by dissociation (Equation (9)):  
\[\text{e}^{-}+\equiv\text{U-OH}\rightarrow\equiv\text{U-OH}^{-} \tag{8}\]  
\[\equiv\text{U-OH}^{-}\rightarrow\text{U-O}^{-}+\bullet\text{H} \tag{9}\]  
H\({}_{2}\) is then formed via \(\bullet\text{H}\) radical recombination, Equation (4). These mechanisms imply that the conditions at the spent fuel surface are reducing due to the radiation-induced production of excess H\({}_{2}\) or other reducing species at the surface. If this is the case, oxidative dissolution does not take place in reducing conditions even without any external input of H\({}_{2}\). The idea that the conditions at the spent fuel surface may be reducing rather than oxidizing has already been proposed by a few authors. [PERSON] et al. [6], after studying the gamma radiolysis of UO\({}_{2}\) in the presence of H\({}_{2}\), concluded that hydrogen not only suppresses oxidation due to radiolytic oxidants but also reduces the extent of surface oxidation observed in either Ar or H\({}_{2}\) atmospheres in the absence of radiation.