## 7 Conclusions  
This work provides an alternative conceptual model to explain the leaching test results involving actual spent nuclear fuel or simulant materials in (confirmed) reducing conditions, which are relevant for deep geologic disposal.  
The key findings are the following: the direct effect of radiation forms radiolytic species (including defects and excited states) in the solid and in the first water layers in contact with its surface; excess H\({}_{2}\) may be produced due to processes occurring at the surface of spent fuel and in confined water volumes involving such species; such species may also play a role in keeping the spent fuel surface in a reduced state.  
Conventional water radiolysis models cannot be used directly to estimate the radionuclide release rates because models and G values are designed for bulk (homogeneous) chemistry in the absence of a solid-liquid interface. The implication of the alternative conceptual model proposed in this work is that the long-term fractional dissolution rate (10\({}^{-7}\) y\({}^{-1}\)) used in most long-term safety assessments is overly estimated because this rate assumes that there is a net UO\({}_{2}\) oxidation rate caused by radiolysis, in contrast with the alternative conceptual model presented here. (Note that the source term for radionuclide release and transport models used in the long-term safety assessment typically involves a long-term fractional dissolution rate for radionuclides embedded in UO\({}_{2}\) and an \"instantly\" released fraction of the radionuclide inventory to take into account the fraction of radionuclides that are typically released shortly after the spent fuel comes in contact with water. Instantly released radionuclides, such as Cs-137, Cl-36, C-14, and I-129, can be found in locations that are easily accessible to water, such as fractures, gaps, or grain boundaries.)  
Because of uncertainties in experiments, in addition to the evolution of the surface area of the fuel in contact with water and long-term self-irradiation effects, adopting a pessimistic long-term fractional dissolution rate and maintaining it at a constant value throughout the assessment period (typically 1 M years) is a cautious approach, at present. This approach is currently used in the source term for long-term safety assessments by spent fuel management organizations, such as Posiva Oy [80], SKB [81], Nagra [82], and NWMO [83].  
This paper also provides suggestions for future work that will yield more information on the interfacial water radiolysis occurring at the spent fuel surface. If it can be proven that the conditions at the spent fuel surface are indeed reducing due to the effect of radiolytic species formed in the spent fuel itself or in the close proximity of the surface, then it will be possible to develop more reliable models for the long-term safety assessment of nuclear fuel disposal.  
Conceptualization; writing--original draft preparation, writing--review and editing: B.P. and J.A.L. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable. Not applicable. Not applicable. Posiva Oy's contribution to this work ([PERSON]'s time and publication fees) is acknowledged. JAL acknowledges the Notre Dame Radiation Laboratory, which is supported by DOE BES Grant Number DE-FC02-04 ER15533. This contribution is NDRL-5332 from the Notre Dame Radiation Laboratory. The authors declare that they have no known competing financial or personal relationships that could have appeared to influence the work reported in this paper. [PERSON] is a staff member of a spent fuel management organization (Posiva Oy) which may be perceived as a non-financial potential conflict of interest.