## 5 Proposed Alternative Conceptual Model
### Processes Occurring at the Time of Interaction between Ionizing Radiation and Matter  
It is well known that the mechanism for water radiolysis proceeds in stages [37; 38], as illustrated in Figure 4 by [PERSON] and [PERSON] [39; 40]. The first stage is the physical stage, during which time there is an energy transfer between the ionizing radiation and the medium. In the case of pure water, an excited water molecule (H\({}_{2}\)O\({}^{*}\)) and an electron-water cation (H\({}_{2}\)O\({}^{+}\)) pair are formed during the physical stage. The physical stage lasts approximately \(10^{-15}\) s.  
The second stage is called the physico-chemical stage (lasting approximately from \(10^{-15}\) s to \(10^{-12}\) s) during which the excited water fragments into \(\bullet\)H and \(\bullet\)OH radicals, in addition to excited O atoms (in triplet or singlet state) and H\({}_{2}\). The electrons solvate, and the transfer of a proton from the water cation to a neighboring water molecule yields the \(\bullet\)OH radical. In summary, this stage leads to formation of the primary radiolytic species (\(\bullet\)H, \(\bullet\)OH, H\({}_{2}\), and hydrated electrons (\(\mathrm{e^{-}}_{\mathrm{aq}}\)).  
During the chemical stage (lasting from \(10^{-12}\) s to approximately \(10^{-6}\) s), the precursor species either escape the non-homogeneous volume where the energy deposition took place or interact with other primary species to form molecular products such as H\({}_{2}\), O\({}_{2}\), and H\({}_{2}\)O\({}_{2}\), in addition to the reformation of H\({}_{2}\)O. Depending on the radiation type, dose and dose rate, and solvents in water, the net balance of these reactions may lead to the recombination of an H\({}_{2}\)O molecule with no net radiolysis effect.  
Finally, the homogeneous stage (from \(10^{-6}\) s onward) is the stage in which radiolysis products become uniformly distributed in water or aqueous solutions and interact with the environment. In the case of spent nuclear fuel, the sources of radiation are the radionuclides embedded in the solid (UO\({}_{2}\)) and many of the precursors and primary species may form in the UO\({}_{2}\) matrix itself, at its surface or in the environment immediately adjacent to the fuel surface. The energy distribution between the solid and the liquid is also important to determine the nature and fate of such radiolytic species. Alpha radiation in water, for example, will deposit most its energy within 13 microns from the surface and will be completely stopped within 35 microns from the surface [19]. The stopping distance of beta radiation in water ranges from a few mm to tens of mm, depending on the energy. Gamma radiation will travel the farthest from the fuel surface and even penetrate through the cladding surrounding the UO\({}_{2}\) pellets and the inner metallic structures of the canister.  
The yield of the various radiolytic products depends on the radiation type and energy, and varies as function of time [41]. The primary yield of a particular radiation product, commonly referred to as the G value, is the amount of radiolysis product produced (or destroyed) per unit of absorbed energy. The G value also depends on the linear energy transfer (LET) of the ionizing particle, which is the amount of energy that the particle transfers to the material traversed per unit distance or stopping power (-dE dx\({}^{-1}\)). For gamma radiation (low LET), the energy is absorbed in a large volume of water, whereas the short range of alpha particles leads to higher LET. The radiolytic yields (G values) obtained from traditional radiolysis studies in homogeneous systems typically reflect the quantity of species that escape from the spurs and tracks, and are able to interact with solutes during the homogeneous stage.