## 6 Outlook on Developments of the Conceptual Model for Spent Fuel-Water Interaction  
The role of the direct effect of the different types of radiation in spent fuel and the interaction with the UO\({}_{2}\) matrix and with the water at the spent fuel interfaces is just starting to be acknowledged. Although there is consensus on the fact that radiolytic species are involved in the processes occurring at 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 no net UO\({}_{2}\) oxidation occurs. Unfortunately, there is no perfect analogue to study the releases of \"aged\" spent fuel. Spent fuel samples currently available have the right physical and chemical properties but still contain a significant amount of beta and gamma radiation, being only a few decades old. Alpha-doped UO\({}_{2}\) samples simulate correctly the level of activity of \"aged\" spent fuel but do not have the right physical and chemical properties. Chemical analogues, such as CeO\({}_{2}\), have the same crystalline (fluorite) structure as UO\({}_{2}\) but no radiation. SIMFUEL, which is synthetic UO\({}_{2}\) containing d-group noble-metal particles, similar to those formed in spent nuclear fuel, also does not emit radiation. All these materials together provide useful insights on the dissolution rate of UO\({}_{2}\), but none individually. Both [PERSON] et al. [32] and [PERSON] et al. [46] showed that metal oxides which have the same crystalline structure can respond differently to radiation (and/or interact differently with water). Therefore, care should also be taken when comparing UO\({}_{2}\) crystalline analogues and drawing conclusions on the energy-water interaction mechanism. Similarly, water radiolysis studies using external sources of ionizing radiation (typically alpha or gamma or accelerated electrons) in the absence of a solid-liquid interface cannot be used to simulate the effect of intrinsic (or self-) irradiation on spent fuel because typically they address the homogeneous stage of water radiolysis and not the interaction between the species formed due to the direct effect of radiation in the solid and those formed at the solid-water interface. For example, [PERSON] and [PERSON] [77; 78] investigated the decomposition of an initial H\({}_{2}\)O\({}_{2}\) solution (5 \(\times\) 10\({}^{-5}\) mol L\({}^{-1}\)) in bulk water under  
Figure 5: Alternative conceptual model for spent fuel–water interaction, modified from [PERSON] et al. [30]. external sources of pure gamma and alpha radiation. In the case of an external source of gamma irradiation, complete H\({}_{2}\)O\({}_{2}\) consumption was observed, confirming the effect of radiolytic species and dissolved H\({}_{2}\) in reducing oxidizing species such as H\({}_{2}\)O\({}_{2}\). However, under an external source of alpha radiation, even if the solution was saturated with H\({}_{2}\) (0.8 mM), the initial H\({}_{2}\)O\({}_{2}\) could not be reduced. In this case, the explanation is that the escape yields of primary radiolytic products from the tracks formed by an external alpha source are too low to interact with solutes in bulk water, such as H\({}_{2}\) and H\({}_{2}\)O\({}_{2}\). In order to interact with the radical species in the alpha particle tracks, much higher concentrations of radical scavengers would be needed, which was not the case in this particular experiment by [PERSON] and [PERSON] [PERSON] and [PERSON] (2018). Alternatively, during bulk water radiolysis using a mixed source of gamma and alpha radiation at 25 \({}^{\circ}\)C (where the alpha particles were produced in situ by neutron capture in a solution containing B\({}^{10}\)), even modest levels of dissolved H\({}_{2}\) (0.8 mM) can suppress the formation of molecular radiolytic oxidants [PERSON] and [PERSON] (2018). The different responses for the alpha radiolysis are mainly due to the energy distribution of the alpha particles.