Effective-Porosity and Dual-Porosity Approaches to Solute Transport in FracturedTuff of the Saturated Zone at Yucca Mountain: Implications for Repository Performance Assessment

摘要

The effective-porosity approach and the dual-porosity approach are examined astwo alternative conceptual models of radionuclide migration in fractured media of the saturated zone at Yucca Mountain. Numerical simulations of one-dimensional radionuclide transport are performed for the domain relevant to repository performance assessment using the two alternative conceptual approaches. The dual-porosity solute transport modeling produces similar results to the effective-porosity model for fracture spacing of less than approximately 1 m and greater than about 200 m, corresponding to values of effective porosity equal to the matrix porosity and the fracture porosity, respectively. For intermediate values of fracture spacing, the dual-porosity approach results in concentration breakthrough curves that differ significantly from the effective porosity approach and are characterized by earlier first arrival, greater apparent dispersion, and lower concentrations at later times. The effective-porosity approach, as implemented in recent peformance assessment analyses of saturated zone transport at Yucca Mountain, is conservative compared to the dual-porosity approach from the perspective of both radionuclide concentrations and generally for travel times.