Parametric and sensitivity analysis of waste isolation in a basalt medium

This report presents the analysis and findings of a numerical modeling study. The purpose of the study was to examine the waste isolation characteristics of the basalt formations. The study basically represents a hydrologic analysis which considers hypothetical release conditions. A parametric and sensitivity analysis approach was used in conjunction with a decision-tree strategy. This approach leads to a systematic and conservative analysis of hydrologic and transport processes. The modeling methodology used in this study is based on the use of numerical models which simulate the coupled processes of groundwater flow, heat transport, and nuclide migration in a fractured-porous media. Applications of these models provided estimates of the changes in the thermal field, groundwater flow patterns, and contaminant plumes as a function of hydrologic and transport parameters. Principal findings of this parametric and sensitivity analysis indicate that the basic hydraulic properties, such as permeability, play a significant role in determining the specific patterns of groundwater flow and nuclide migration. With regard to nuclide migration, however, the magnitude of the initial inventory and the sorption properties generally determine the rate and extent of nuclide movement. Heat transport coupling to groundwater dominate the flow pattern in early times by virtue of buoyancy effects. Long-term simulations, i.e., 250,0000 years, indicate that for the most conservative conditions the radionuclides released from the repository are contained in the host basalt formation. This suggests that the natural barriers can achieve a high degree of waste isolation