The bentonite buffer long-term integrity is of significant interest in the performance assessment (PA) of nuclear waste disposal. This study aims at understanding how the initial geochemical parameters affect long-term chemical properties within the buffer, which will subsequently affect the transport. Using coupled thermal-hydrological-chemical (THC) models for migration of U(VI) in a generic repository, we performed a global sensitivity analysis (GSA) to identify the influence of each parameter on the temporal evolution of a spatially averaged distribution coefficient for the entire buffer. Such an analysis can be used in a repository-scale PA. In this work, we used the TOUGHREACT software to model coupled THC processes in a generic clay repository with bentonite buffer. In this model, U(VI) is released from a canister via schoepite dissolution, which is assumed to occur 1000 years after closure. U(VI) migrates through the bentonite buffer affected by two-site protolysis non-electrostatic surface complexation and cation exchange. GSA results showed that adsorption density on smectite, pH, volume fractions of smectite, calcite, Ca2+ aqueous concentration all play a significant role in U(VI) transport, since roughly 80% of adsorbed U(VI) is absorbed by smectite, and Ca2+ affects the aqueous complexation with U(VI). This work demonstrates the complex process models potential usefulness that can be transferred to the PA model. It also provides information needed to proceed with the development of a reduced-order model, which has the potential to optimize repository designs, site characterization, performance confirmation.

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We developed accurate and efficient surrogates for multi-physics simulations • The surrogates' accuracy is greatly enhanced by clustering of training data • The surrogates can be used in system-level assessment of a nuclear waste repository