Publications

In this paper, a design methodology is presented for assessing drift stability for a potential high-level radioactive waste repository. Excavation stability 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. Requirements for the design, site conditions and stresses are considered in the methodology. Methods for evaluating empirical and analytical results in order to estimate ground support requirements are outlined.

Thermal and mechanical models for intact and jointed rock mass behavior are being developed, verified, and validated at Sandia National Laboratories for the Yucca Mountain Project. Benchmarking is an essential part of this effort and is the primary tool for verifying engineering software used to solve thermomechanical problems. This report presents the results of the second phase of the first thermomechanical benchmark exercise. In the first phase of this exercise, three finite element codes for nonlinear heat conduction and one coupled thermoelastic boundary element code (HEFF) were used to solve the thermal portion of the benchmark problem. The boundary element code HEFF was used in this exercise because it calculates a solution to the coupled thermal/elastic problem using an approximate analytical method and, thus, provides a means of comparing the finite element solutions with a solution obtained by an independent method. The results from the thermal analysis were then used as input to the second phase of the analysis, which consisted of solving the structural portion of the benchmark problem using a linear elastic rock mass model. Five different structural codes, JAC, SPECTROM-31, VISCOT, and HEFF, were used by the participants in this portion of the study. The problem solved by each code was a two-dimensional idealization of a series of drifts with the approximate dimensions of the proposed design for vertical emplacement of nuclear waste at Yucca Mountain. 6 refs., 74 figs., 4 tabs.

The Yucca Mountain Project (YMP) 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 (UNEs). 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.