Numerical modeling of flow and transport processes in a fractured-porous rock system

The feasibility of using underground repositories for terminal waste storage depends on the basic isolation capability of the candidate geologic medium. In the case of a hardrock geology, such as basalt, the degree of waste isolation will be determined by the response of the water-rock system to both natural and repository-induced perturbations. Numerical models capable of describing hydrological and geomechanical processes provide a basis for assesssing the waste isolation performance of the host medium. Numerical models were recently developed for specific use in modeling the near-field processes in fractured-porous rock systems. Transport of heat through the water-rock system is modeled in a fully coupled manner with groundwater flow. Migration of dissolved nuclides is modeled by accounting for the transport processes of advection, dispersion/diffusion, sorption, and chain decay. A Galerkin finite-element approach is used in conjunction with a Newton-Raphson method to solve the set of governing equations. The numerical models were recently applied in a parametric and sensitivity modeling study to evaluate the basic waste isolation characteristics of the Columbia River basalts beneath the Hanford Site, near Richland, Washington. In this paper, selected simulation results are presented which show the time-dependent response of the basalt waste isolation system over long-term periods