Geomechanical response to thermal loading at the proposed Yucca Mountain repository for highlevel nuclear waste may result in changes in rock-mass hydrological properties within laterally discontinuous zones in the repository horizon. The altered zones would be characterized by increased horizontal permeability and their development will depend on the thermal loading, rock-mass thermal and mechanical properties, and design features such as ventilation that may affect heat transfer into the host rock. Lateral diversion of moisture within or around the altered zones can be expected and would result in a redistribution of the cross-repository moisture flux. Lateral flow would be directed down-dip of the site-scale stratigraphy. Therefore, cross-repository flux would be reduced at the up-dip ends of the thermal-mechanical altered zones and increased at the down-dip ends. Consequently, parts of an emplacement drift close to the down-dip end of a thermal-mechanical altered zone can be expected to experience elevated percolation flux.
INTRODUCTION
The host rock mass for the proposed repository at Yucca Mountain may undergo changes in hydrological properties owing to the geomechanical response of the rock mass to the heat generated by radioactive decay of nuclear waste. Changes in hydrological properties should be considered because of their potential effect on the amounts of water seepage (percolation flux) through the repository horizon. An understanding of the temporal and spatial distributions of percolation flux is essential to predict the occurrence and magnitudes of seepage into the eraplacement drifts, and, therefore, the onset and rates of waste-package corrosion and the Wansport of mdionuclides in aqueous states to the saturated zone.
Thermally induced geomechanical response (in addition to responses induced by excavation and potential seismic loading) may induce changes in fracture aperture and, hence, fracture porosity, permeability, and, possibly, capillarity. Thermalmechanical response will be controlled by the geometry of the emplacement area, thermal and mechanical properties of the rock mass, nature and distribution of waste packages, and other design features (such as ventilation) that may affect heat transfer from the waste packages to the rock mass. The emplacement geometry [Civilian Radioactive Waste Management System (2000)] consists of a horizontal array of drifts at a depth of about 300 m below the ground surface. For a typical drift within the array, thermal expansion of the surrounding rock would be fully suppressed laterally but a limited amount of vertically upward expansion can occur because of free movement at the ground surface (Figure 1).
