ABSTRACT

INTRODUCTION

A thorough evaluation of the geologic material being considered must be conducted to determine the feasibility of constructing a large-scale underground facility in a deep geologic formation. The design and construction of a nuclear waste repository in basalt, due to the unprecedented nature of the project, adds significantly to the need for thoroughness of this evaluation and mandates that geo-engineering data and analysis be of the highest quality. The investigations required to collect the data encompass in situ measurements and observations, as well as laboratory and field tests. Mathematical (numerical) models, which are required to predict the long-term performance of the repository structure, must be developed to evaluate rock mass behavior at canister, room, and repository scale and eventually must be validated through in situ tests and monitoring. The information generated from these geo-mechanics investigations is essential for assessing site suitability and will be utilized in the development of the repository design, the engineered barrier design, and the evaluation of repository performance. The Hanford Site near Richland, Washington is one of nine sites throughout the nation being investigated by the U.S. Department of Energy as a possible location for the first high-level nuclear waste repository in deep geologic formations. The process of site characterization is currently in progress. If the site is nominated by the U.S. Department of Energy, an extensive program will be undertaken, the culmination of which will be a decision on the suitability of the site for waste disposal. There are a multitude of elements to the proposed site characterization program including an examination of the natural barriers to radionuclide migration, i.e., the geologic, geo-mechanical, geochemical, and hydrologic characteristics of basalt formations in the Pasco Basin. In the final analysis, the site must be capable of limiting the release of radionuclides to the accessible environment over a period of 10,000 yr in accordance with U.S. Environmental Protection Agency regulations (EPA, 1982). While the hydrologic characteristics of the basalts forming the primary lithologic sequence of the Pasco Basin are crucial to the isolation process, the importance of the geo-mechanics characteristics should not be understated. The rock structure must be such that a safe and stable environment for waste canister emplacement and, if necessary, retrieval is created. The repository must maintain its structural integrity at elevated temperatures during the operational period and after closure. Therefore, it is those geo-mechanics characteristics which will be the focus of this paper.

THE REPOSITORY FACILITY

As currently conceived (RKE/PB, 1983), the repository is to consist of a single level of parallel drifts approximately 61 m apart between which large diameter (76 cm) boreholes will be drilled for emplacement of a string of waste canisters. The facility is to receive commercial high-level waste and spent fuel over a 20 yr period and is to remain open for another 30 yr to provide for any required waste retrieval prior to backfilling of all openings. The area1 extent of the repository is expected to be approximately 3.2 km by 3.2 km. As many as five shafts will provide access, ventilation, and other functional requirements.

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