In this study, we develop a mechanism-based model to take into account most of the important processes that may influence the dissolution behavior of spent fuel and subsequently the release behavior of nuclides within a defective canister in a final repository for high-level nuclear waste. The model is, in essence, a redox-controlled reactive transport model that provides a description of the mass transport of multiple species involved in both local equilibrium and kinetically controlled reactions in the system. The complexity of the kinetics of the various redox reactions involved and the requirement of the long-term prediction, however, make numerical implementation of the fully coupled model computationally inefficient. A series of scoping calculations was performed to highlight the local characteristics and behaviors of the system, and to provide a basis for refinement of the reactive transport model. The results indicate that the rapid buildup of hydrogen within the system is mainly attributed to corrosion of the cast-iron insert that primarily occurs under anaerobic conditions, rather than to radiolysis of water. The system that is rapidly in equilibrium with 50 bar hydrogen would then keep pH constant throughout the system. In addition, simulations suggest that reduction of dissolved hexavalent uranium by ferrous iron adsorbed onto the corrosion products and by dissolved H-2 are the most important mechanisms to retard the release of uranium out of the canister. More importantly, it is found that the pseudo stationary state approximation may well be applied to the system. This greatly simplifies the numerical implementation of the reactive transport model. References: 31
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