Challenge. Determine importance of various physical processes (e.g., infiltration, percolation, unsaturated zone flow, fracture-matrix flow, heat generation, radionuclide transport, etc.) on repository performance

Solution. The Yucca Mountain site is located approximately 100 miles northwest of Las Vegas, Nevada. The site hydrogeology consists a thick unsaturated zone comprised primarily of volcanic deposits (tuffaceous rocks). The complexity of the Yucca Mountain site necessitated the use of sophisticated numerical models to evaluate the physical processes at work and their implications on repository design and performance. INTERA’s process modeling efforts included developing a predictive model for repository scale variations in percolation flux due to uncertainties in subsurface hydrologic characterization, applying reliability theory for uncertainty propagation in unsaturated flow systems, and developing a semi-analytical model to analyze steady-state infiltration into a layered formation with uncertain parameters. Methods for modeling non-equilibrium fracture-matrix flow in unsaturated media were developed and applied to a site-scale model of the unsaturated zone at Yucca Mountain to investigate the sensitivity of mountain-scale moisture movement to uncertainties in hydrologic properties, alternate conceptual models of fracture-matrix interaction, and multiple infiltration scenarios. INTERA developed a mountain-scale 3-dimensional thermo-hydrologic model to examine the duration and magnitude of heat driven perturbations to the ambient system. Two-dimensional cross-sectional models were used to investigate the impacts of local scale heterogeneities and thermo hydrologic changes within waste emplacement tunnels. A scaling methodology was developed to account for edge-cooling effects in the proposed repository. Three-dimensional drift-scale models were developed to predict thermo-hydrologic conditions in waste-emplacement drifts, and we conducted a benchmark study of drift-scale heat- and fluid-flow using the integrated finite difference code TOUGH2 and the finite element code FEHM. We also developed a sub-repository-scale thermo-hydrologic model to predict temperature and saturation changes induced by the emplacement of heat-generating radioactive waste. To evaluate the thermal-hydrologic-mechanical (THM) effects in the fractured tuff associated with heat generation from waste canisters, we performed scoping calculations using a coupled reservoir-geomechanical model. Results from our process modeling efforts were incorporated into a total system model to evaluate the ability of the overall waste disposal system to meet regulatory standards.