Site characterization of the Yucca Mountain disposal system for spent nuclear fuel and high-level radioactive waste

• Progression of site characterization through four phases is described for a geological repository at Yucca Mountain.
• Well testing from the surface to determine hydraulic flow properties in the saturated and unsaturated zones is summarized.
• Investigation of infiltration in a desert climate is summarized.
• Underground testing to evaluate coupled hydrologic–thermal processes and determine seepage into disposal drifts is summarized.

This paper summarizes the investigations conducted to characterize the geologic barrier of the Yucca Mountain disposal system. Site characterization progressed through (1) non-intrusive evaluation and borehole completions to determine stratigraphy for site identification; (2) exploration from the surface through well testing to evaluate the repository feasibility; (3) underground exploration to study coupled processes to evaluate repository suitability; and (4) reporting of experimental conclusions to support the repository compliance phase. Some of the scientific and technical challenges encountered included the evolution from a small preconstruction characterization program with much knowledge to be acquired during construction of the repository to a large characterization program with knowledge acquired prior to submission of the license application for construction authorization in June 2008 (i.e., the evolution from a preconstruction characterization program costing <$0.04×109 as estimated by the Nuclear Regulatory Commission in 1982 to a thorough characterization, design, and analysis program costing $11×109—latter in 2010 constant dollars). Scientific understanding of unsaturated flow in fractures and seepage into an open drift in a thermally perturbed environment was initially lacking, so much site characterization expense was required to develop this knowledge.