## 5 Proposed Alternative Conceptual Model
### Processes Occurring in the Bulk of UO\({}_{2}\) (Direct Effect of Ionizing Radiation)  
Ionizing radiation is known to create electronic defects (such as \"holes\" formed by the release of an electron from a valence band), or excited states such as polarons or excitons (bound excited states of electrons and holes), and phonons (vibrating atoms in a crystalline structure). Additionally, ionizing radiation can produce Frenkel-type defects created by the displacement of an oxygen atom in the UO\({}_{2}\) lattice to form an interstitial-vacancy pair. They are especially common with heavy ions (such as alpha particles) because of the large relative momentum transfer possible in nuclear collisions. The local density of Frenkel defects in a heavy ion track is much greater than that found in gamma radiolysis and  
Figure 4: Processes occurring upon the interaction of ionizing radiation and a water molecule leading to the production of radiolytic species in solution. Figure prepared based on schemes for the radiolysis of water reported in refs [39; 40].  
can lead to considerable self-trapping of excitons, the potential precursor to H\({}_{2}\)[42; 43]. A local increase in the temperature occurs along the high-energy ion path. This temperature increase likely induces an enhanced defect migration leading to defect rearrangement [44].  
Defect rearrangement causes point defects to coalesce into a lower energy defect structure (dislocation loops), as observed in self-irradiated UO\({}_{2}\), or alpha-doped UO\({}_{2}\) or PuO\({}_{2}\) samples [44; 45]. The local density of Frenkel defects in a heavy ion track is much greater than that found in gamma radiolysis and can lead to considerable self-trapping of excitons, which are bound excited states of electrons and holes, the potential precursor to H\({}_{2}\).  
Furthermore, [PERSON] et al. [46] studied the response of UO\({}_{2}\), PuO\({}_{2}\), and ThO\({}_{2}\) to an external source of alpha particles (He ions). [PERSON] et al. [46] propose that there is a coupling between electronic defects and phonons in UO\({}_{2}\) and PuO\({}_{2}\), whereas alpha irradiation only causes electronic defects in ThO\({}_{2}\). The authors suggest that the alpha-induced irradiation defects may be involved in chemical reactions at the UO\({}_{2}\)-water interface. In doing so, the UO\({}_{2}\) boundary layer acts as a sink and governs the annealing of defects.  
[PERSON] et al. [47] and [PERSON] et al. [48] also highlighted the importance of atomistic defects during the dissolution of uranium mixed oxides (U-Pu)\({}_{x}\)O\({}_{y}\) and cerium oxides (CeO\({}_{2}\)), respectively. The authors showed that the high leaching rate of the studied metal oxides was related to the reduced chemical durability of the materials due to the presence of defects in the crystalline lattice, such as oxygen vacancies.  
[PERSON] et al. [11] suggest that the neutralization of the radiolytic oxidants produced near the oxide surface by alpha radiolysis may be due to oxygen vacancies created on the surface of UO\({}_{2}\)(s), which likely contributes to the release of hydrogen atoms (\(\bullet\)H) through the dissociative adsorption of a water molecule. The authors could not rule out that this or other surface processes contribute to the orders of magnitude lower homogeneous radiolytic yields by alpha particles emitted by UO\({}_{2}\)(s) surfaces, compared to the yields measured with an external source of alpha radiation.  
These findings indicate that species formed due to self-irradiation of UO\({}_{2}\) (oxygen vacancies, electronic defects, or excited species) may play a significant role in the chemical reactions occurring at or near the fuel matrix-water interface.