U.S. Department of Energy, Office of Civilian Radioactive Waste Management, Yucca Mountain Site Characterization Project, under Contract No. DE-AC04-76DP00789
Authors: M. P. Hardy (AAI), S. J. Bauer (Sandia National Laboratories), and C. E. Brechtel and R. R. Goodrich (AAI)

The Yucca Mountain Project (YMP), managed by the Nevada Operations Office of the U.S. Department of Energy (DOE), is examining the feasibility of siting a repository for high-level nuclear waste at Yucca Mountain, on and adjacent to the Nevada Test Site (NTS). The proposed repository will be excavated in the Topopah Spring Member, which is a moderately fractured, unsaturated, welded tuff. Excavation stability will be required during construction, waste emplacement, retrieval (if required), and closure to ensure worker safety. The subsurface excavations will be subject to stress changes resulting from thermal expansion of the rock mass and seismic events associated with regional tectonic activity and underground nuclear explosions. Analyses of drift stability are required to assess the acceptable waste emplacement density, to design the drift shapes and ground support systems, and to establish schedules and cost of construction. This paper outlines the proposed methodology to assess drift stability and then focuses on an example of its application to the YMP repository drifts based on preliminary site data. Because site characterization activities have not begun, the database currently lacks the extensive site-specific field and laboratory data needed to form conclusions as to the final ground support requirements. This drift design methodology will be applied and refined as more site-specific data are generated and as analytical techniques and methodologies are verified during the site characterization process. The work presented in this paper is part of an ongoing effort to improve the interface between analysis and design, to identify weaknesses in the available data, and to assist the site geomechanics characterization effort in identifying critical parameter needs. The numerical values used in the example, although based on data in the Reference Information Base (DOE 1989), are not necessarily representative of the site nor useful for design purposes.

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