In 1983, high-level radioactive waste repository performance requirements related to groundwater travel time were defined by NRC subsystem regulation 10 CFR 60.113. Although the DOE is not presently attempting to demonstrate compliance with that regulation, understanding of the prevalence of fast paths in the boundwater flow system remains a critical element of any safety analyses for a potential repository system at Yucca Mountain, Nevada. Therefore, this analysis was performed to allow comparison of fast-path flow against the criteria set forth in the regulation. Models developed to describe the conditions for initiation, propagation, and sustainability of rapid groundwater movement in both the unsaturated and saturated zones will form part of the technical basis for total-system analyses to assess site viability and site licensability.This study is an enhancement of previous groundwater travel time analyses. In it the authors attempt to define the appropriate conceptual model for assessing fast-path flow. The conceptual model thus allows disequilibrium flow in the fractures and in the matrix. This conceptual model also allows for spatial and temporal variability of infiltration from the ground surface. In addition, the natural heterogeneity in hydrologic properties and the uncertainty in the values for those properties is expected to play an important role in allowing controlling rapid movement of groundwater. Because of this uncertainty in the distribution of rock properties, a probabilistic approach has been used in the unsaturated zone. Geostatistical simulation techniques, constrained by a geologic framework model, are used to create multiple realizations of the unsaturated domain. Numerical modeling of steady-state groundwater flow in these heterogeneous, two-dimensional domains is accomplished using the TOUGH2 code. A particle tracking method is used to simulate the advective and dispersive movement of groundwater in the simulated flow field and to calculate travel times from the repository to the water table. Numerical modeling of steady-state groundwater flow in the saturated zone is performed within a deterministic, three-dimensional model domain representing the upper 250 m of the saturated zone. Advective and dispersive transport are simulated by particle tracking from the water table to the five km limit of the accessible environment.The results of the groundwater flow modeling provide insights into the implications of the conceptual models employed, the sensitivity of modeling results to specific parameters and assumptions, and distributions of groundwater travel time for a set of base case calcu1,ations. Values of matrix saturation simulated by the unsaturated flow modeling compare favorably with measured matrix saturations from boreholes near the modeled cross-sections. Particle travel times in the unsaturated zone are particularly sensitive to infiltration rates, fracture-matrix connectivity, and fracture frequency. Increased infiltration rate, in concert with reduced fracturematrix co...

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