Effect of heterogeneity in fracture permeability on the potential for liquid seepage into a heated emplacement drift of the potential repository.

A numerical model was used to investigate the effect of spatial variability in fracture permeability on liquid seepage and moisture distribution in the vicinity of a waste emplacement drift in the unsaturated zone (UZ) of Yucca Mountain. The model is based on a two-dimensional, cross-sectional, dual-permeability model of the unsaturated zone at Yucca Mountain and uses a stochastic approach to investigate the effect of small-scale heterogeneous features. The studies were conducted using one uniform fracture permeability case, three realizations of stochastically generated fracture permeability, one discrete permeability feature case, and one increased ambient liquid flux case. In all cases, the models predict that completely dry drift conditions will develop above and below the drift in 10-100 years and remain dry for 1000-2000 years. During this period, the models predict no seepage into drifts, although liquid flux above the drifts and within the drift pillars may increase by up to two orders of magnitude above ambient flux. This is because the heat released by the emplaced waste is sufficient to vaporize liquid flux of one to two orders of magnitude higher than present-day ambient flux for over 1000 years. The results also show that unsaturated zone thermal-hydrological (TH) models with uniform layer permeability can adequately predict the evolution of seepage and moisture distribution in the rock mass surrounding the repository drifts. The models further show that although variability in fracture permeability may focus and enhance liquid flow in regions of enhanced liquid saturation (due to condensation above the drifts), vaporization and vapor diffusion can maintain a dry environment within the drifts for thousands of years.