Calculation of Permeability Change Due to Coupled Thermal-Hydrological-Mechanical Effects

The purpose of this calculation is to provide a bounding estimate of how thermal-hydrological-mechanical (THM) behavior of rock in the region surrounding an emplacement drift in a Monitored Geologic Repository subsurface facility may affect the permeability of fractures in the rock mass forming the region. The bounding estimate will provide essential input to performance assessment analysis of the potential repository system. This calculation also supports the Near Field Environment Process Model Report (NFE PMR) and will contribute to Site Recommendation. The geologic unit being considered as a potential repository horizon at Yucca Mountain, Nevada lies within a fractured, densely welded ash-flow tuff located in the Topopah Spring Tuff member of the Paintbrush Group. Fractures form the primary conduits for fluid flow in the rock mass. Considerable analysis has been performed to characterize the thermal-hydrologic (TH) behavior of this rock unit (e.g., CRWMS M and O 2000a, pp. 83-87), and recently the dual permeability model (DKM) has proved to be an effective tool for predicting TH behavior (CRWMS M and O 2000a). The DKM uses fracture permeability as a primary input parameter, and it is well known that fracture permeability is strongly dependent on fracture deformation (Brown. 1995). Consequently, one major unknown is how deformation during heating and cooling periods may change fracture permeability. Opening of fractures increases their permeability, whereas closing reduces permeability. More importantly, shear displacement on fractures increases their permeability, and fractures undergoing shear are likely to conduct fluids. This calculation provides a bounding estimate of how heating and cooling in the rock surrounding an emplacement drift and the resulting mechanical deformation may affect the fracture permeability of the rock