Three-Dimensional Thermal and Mechanical Scoping Calculations for Underground Disposal of Nuclear Waste in Shale

摘要

Thermal and mechanical scoping calculations were performed to determine the responses of two different shales to the underground disposal of nuclear waste. The geometry under consideration was a three-dimensional model for waste emplacement in a conventional room-and-pillar configuration. The first medium, high-illite shale with no expandable clays, exhibits a positive and nearly linear thermal expansion with increasing temperature. The second medium, eastern Pierre shale with abundant expandable clays, however, will dehydrate and volumetrically contract several percent upon heating to the local boiling point of the pore water. Results of the thermal calculations are presented which show that only the very-near-field rock temperatures within the boiling isotherm are highly sensitive to the rock mass thermal conductivity and, hence, the expandable clay content. The mechanical calculations for the high-illite shale show no evidence of post-excavation instability, and no change in the stress field as a result of waste emplacement. The mechanical response of the eastern Pierre shale, however, is characterized by zones of volumetric contraction and joint opening located within the boiling isotherm and resulting directly from the dehydration shrinkage. (ERA citation 06:019886)