Unsaturated flow modeling in performance assessments for the Yucca Mountain disposal system for spent nuclear fuel and high-level radioactive waste

• Progression of modeling of infiltration, percolation, and seepage conducted is described for a geological repository at Yucca Mountain.
• Progression from 1-D in single equivalent to 3-D model of percolation in dual permeability continuum is described.
• Introduction of an infiltration boundary condition in 1998 and the refinement for evaluating uncertainty for the license application is described.
• Introduction of a seepage module that included calibration to in-site measurements and separating uncertainty and variability is described.

This paper summarizes the progression of modeling efforts of infiltration, percolation, and seepage conducted between 1984 and 2008 to evaluate feasibility, viability, and assess compliance of a repository in the unsaturated zone for spent nuclear fuel and high-level radioactive waste at Yucca Mountain, Nevada. Scientific understanding of infiltration in a desert environment, unsaturated percolation flux in fractures and matrix of the volcanic tuff, and seepage into an open drift in a thermally perturbed environment was initially lacking in 1984. As understanding of the Yucca Mountain disposal system increased through site characterization and in situ testing, modeling of infiltration, percolation, and seepage evolved from simple assumptions in a single model in 1984 to three modeling modules each based on several detailed process models in 2008. Uncertainty in percolation flux through Yucca Mountain was usually important in explaining the observed uncertainty in performance measures:cumulative release in assessments prior to 1995 and individual dose, thereafter.