This chapter focuses on radioactive waste repository design. Underground storage in a mined repository is the preferred option for long-term disposal of high-level nuclear waste materials from the commercial nuclear power electric generating plants and for defense high-level wastes. The United States Department of Energy is responsible for selecting a site and designing and licensing the repository. The chapter presents methodology criteria to Yucca Mountain for assessing repository drift stability, ground support, and rock reinforcement requirements. Although empirical and observational methods are a part of the overall design methodology, their contribution to drift design is limited at this stage of the repository design because of the lack of comparable excavation experience and observation of drift performance. The final drift design consists of the drift dimensions, shape, and the ground support/reinforcement system. Not all repository drifts can have the same dimensions and shapes, because the function of a drift can vary. Similarly, geological conditions are expected to vary through the repository and hence, the ground support and rock reinforcement system can vary depending on ground conditions.

The Yucca Mountain Project, 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. Excavation stability will be required during construction, waste emplacement, retrieval (if required), and closure, covering a period of approximately 100 years. In order to incorporate a means of evaluating excavation stability in the design process, a drift design methodology has been developed. This methodology uses both empirical and analytical methods in conjunction with detailed descriptions of site conditions to evaluate a proposed design. At present, the emphasis is on analytical numerical methods because of the limited experience in tuff at elevated temperatures. Proposed methods for analysis of systematically jointed, isotropically jointed, and widely spaced, discretely jointed rock masses are described.

Excavation stability in an underground nuclear waste repository is required during construction, emplacement, retrieval (if required), and closure phases to ensure worker health and safety, and to prevent development of potential pathways for radionuclide migration in the post-closure period. Stable excavations are developed by appropriate excavation procedures, design of the room shape, design and installation of rock support reinforcement systems, and implementation of appropriate monitoring and maintenance programs. In addition to the loads imposed by the in situ stress field, the repository drifts will be impacted by thermal loads developed after waste emplacement and, periodically, by seismic loads from naturally occurring earthquakes and underground nuclear events. A priori evaluation of stability is required for design of the ground support system, to confirm that the thermal loads are reasonable, and to support the license application process. In this report, a design methodology for assessing drift stability is presented. This is based on site conditions, together with empirical and analytical methods. Analytical numerical methods are emphasized at this time because empirical data are unavailable for excavations in welded tuff either at elevated temperatures or under seismic loads. The analytical methodology incorporates analysis of rock masses that are systematically jointed, randomly jointed, and sparsely jointed. In situ thermal and seismic loads are considered. Methods of evaluating the analytical results and estimating ground support requirements for all the full range of expected ground conditions are outlines. The results of a preliminary application of the methodology using the limited available data are presented. 26 figs., 55 tabs.

An understanding of the thermal and stress environment in the vicinity of repository openings is important for preclosure performance considerations and worker health and safety considerations for the proposed high-level radioactive waste repository at Yucca Mountain. This paper presents the results of two and three dimensional numerical analyses which have determined the thermal and stress environments for typical repository openings. In general, it is predicted that openings close to heat sources attain high temperatures and experience a significant stress increase. Openings away from heat sources experience more uniform temperature changes and experience a stress change which results in part from a far-field thermal loading. 6 refs., 6 figs., 3 tabs.