The Cohassett basalt flow underlying the Hanford Site in Eastern Washington is being investigated by the U.S. Department of Energy to assess the feasibility of constructing the nation's first nuclear waste repository in the flow. Development of constitutive laws of the basalt is necessary for modeling studies used to evaluate the repository design and to evaluate the ability of the repository to contain radioactive waste for several thousands of years. This paper evaluates four analytical approaches that have been used in previous analyses of the repository in basalt and presents a strategy for further development of constitutive models.
The Cohassett basalt flow underlying the Hanford Site in Eastern Washington State is one of three rock types being investigated by the U.S. Department of Energy for the nation's first nuclear waste repository. A constitutive model of the basalt rock mass is a critical element for the design of a licensable repository for nuclear waste. Repository design is different in many aspects from the design of conventional underground structures. The expected life of the repository is exceptionally long; openings must be maintained for more than 70 years prior to closure of the repository and the ability for the rock to contain nuclear waste must be maintained for several thousand years after closure. During its life, the repository will be subjected to high thermal loads generated from the decay of radionuclides. Also, the repository must be designed to limit the migration of radionuclides in a manner to satisfy strict regulatory requirements. The repository host rock plays a major role in maintaining isolation capability, thus its constitutive behavior needs to be understood. Repository design is of an iterative nature and evolves through various stages of conceptual design to a final design. The design process provides information required to apply for a license to construct a nuclear waste repository. Four design phases have been identified: conceptual design, Conceptual Design for the Site Characterization Plan (SCP/CD), Advanced Conceptual Design and License Application Design. Each design serves specific purposes determined by program requirements. An architect/engineer, Kaiser Engineers/ Parsons Brinckerhoff, Quade and Douglass, performs the actual design of the repository. Repository design generally used standard design methodolgies and tools An assessment of the design is made by the U.S. Department of Energy through its prime contractor, Rockwell Hanford Operations. The design assessment process is intended to demonstrate that the design provided by the architect/engineer is functional under a variety of environmental conditions to which the facility may be subjected. Design assessment provides the basis for establishing a high degree of confidence in the ability of the repository to meet applicable federal regulations. Concurrent with the design effort, site characterization activities (from surface exploration to in situ testing) provide continuous improvements in understanding the behavior of the natural environment at the proposed repository site. In order to assess the performance of the design under forseeable conditions, this data will be utilized in an extensive modeling and analysis program. Because constitutive models of the rock mass are a key element in these analyses, model development is an integral part of the site characterization program.