A probabilistic assessment of the possible formation of key blocks within the potential repository horizon has been performed based on the orientations of discontinuities present in the Exploratory Studies Facility at Yucca Mountain, Nevada. A primary objective of the key block analysis is to provide a statistical description of block sizes formed by fractures around the emplacement drifts openings. This analysis has considered various emplacement drift orientations, examined unsupported drifts, and applied both static and seismic loading conditions. The use of key block theory provides a practical tool for assessing rock mass behavior surrounding emplacement drift openings. The results from this study provide block size data to support waste package design and provide degraded drift profile data to support other repository performance assessment analyses.