Geologic, geochemical, rock mechanics, and hydrologic characteristics of candidate repository horizons

The objective of the Basalt Waste Isolation Project (BWIP) is to determine the feasibility of constructing a nuclear waste repository in basalt (NWRB) on the Hanford Site. Studies conducted to date indicate that an NWRB is feasible and would meet performance requirements with a significant safety margin. The two most promising candidate repository horizons for an NWRB are the middle Sentinel Bluffs and the Umtanum flows. Both of these flows are laterally continuous and have thicknesses of competent rock adequate to accommodate a repository. Significant geologic differences between the two flows are their depth, total thickness, and variability of flow top thickness. These differences will be considered in selecting one of the two flows for breakout from an exploratory shaft. The geochemical characteristics of both the middle Sentinel Bluffs flow and the Umtanum flow favor long-term isolation of radionuclides by providing an environment in which canister corrosion rates and solubility of many radionuclide-bearing solids is relatively low. The low permeabilities of the interiors of these two flows are also favorable for long-term performance of a repository. Finally, the interiors of both horozons consist of highly competent rock; preliminary design analysis based on the mechanical properties of these rocks indicate that stable openings can be excavated in both candidate repository horizons. 6 figures, 5 tables