The design of rock support for underground nuclear waste repositories requires consideration of special construction and operation requirements, and of the adverse environmental conditions in which some of the support is placed. While repository layouts resemble mines, design, construction and operation are subject to quality assurance and public scrutiny similar to what is experienced for nuclear power plants. Exploration, design, construction and operation go through phases of review and licensing by government agencies as repositories evolve. This paper discusses (1) the various stages of repository development; (2)the environment that supports must be designed for; (3) the environmental effects on support materials; and (4) alternative types of repository rock support. Much of the information presented in this paper was derived from work conducted for Basalt Waste Isolation Project. Most of the information, however, would apply to other hard rock repositories.
Repository development can be characterized by five general stages: exploration and design, excavation, waste emplacement, retrievability period, and backfilling. It is helpful to understand these phases of development to see where and when rock supports are required and how the design is verified. The description that follows is simplified. Details have been omitted, and future plans may not conform to this scenario. To prevent radionuclides from entering the biosphere, repositories will be located at relatively great depths. Geologic environments currently considered include domed and bedded salt, tuff, basalt and crystalline rock such as granite. The Department of Energy is conducting studies to determine which type of rock is best suited for construction of the first nuclear waste repository in the United States. The initial stages of exploration must provide adequate characterization for site selection and design with a minimum of boreholes above or through the repository horizon. Borings could provide a path for radionuclide migration. Therefore, surface mapping and regional geologic and hydrologic information are used, together with few borings, to select an area to be considered. A limited geotechnical exploration is then conducted to determine relevant geologic and hydrologic parameters for conceptual design. If the site is deemed fit for further consideration, an exploratory shaft is sunk. This shaft would probably become part of the repository, if the site is selected. From the exploratory shaft boreholes are drilled outward, and tests are conducted in relevant geologic horizons. At the repository depth, short drifts are excavated, and testing and monitoring programs are conducted to investigate rock mass properties, excavation methods, drift size and shapes, response to thermal loading, and acceptable rock support methods. Extensive instrumentation is used during this phase of development to acquire data and to test instrumentation and monitoring methods proposed for use in the repository. The information is analyzed and a design is prepared for construction of the repository. Only after approval by the Nuclear Regulatory Commission (NRC) can construction begin. During the exploratory stage, rock support design will have been conceptualized, developed (if required), installed, tested, monitored and improved for use in the repository. Long-term reliability and response to thermal loading due to waste emplacement will have been assessed to the degree required but not ultimately verified.
