Micromechanical fracture modeling on underground nuclear waste storage: Coupled mechanical, thermal, and hydraulic effects

Abstract

Coupling effects between thermal, hydraulic, chemical and mechanical (THCM) processes for rock materials are one of major issues in Geological engineering, Civil engineering, Hydrology, Petroleum engineering, and Environmental engineering. In all of these fields, at least two mechanisms of THCM coupling are considered. For an example, thermal, hydraulic, and mechanical coupling effects are important in Geological engineering and Civil engineering. The THM coupling produces effects on underground structures, since the underground structures are under influences of geothermal gradient, groundwater, gravitational stresses, and tectonic forces. In particular, underground repository of high-level nuclear waste involves all four of the THCM coupling processes. Thermo-hydro-mechanical coupling model for fractured rock media has been developed based on micromechanical fracture model [Kemeny 1991, Kemeny & Cook 1987]. The THM coupling model is able to simulate time- and rate-dependent fracture propagation on rock materials, and quantify characteristics of damage by extensile and shear fracture growth. The THM coupling model can also simulate coupled thermal effects on underground structures such as high-level nuclear waste repository. The results of thermo-mechanical coupling model are used in conducting a risk analysis on the structures. In addition, the THM coupling model is able to investigate variations of fluid flow and hydraulic characteristics on rock materials by measuring coupled anisotropic permeability. Later, effects of chemical coupling on rock materials are investigated and modified in the THM coupling model in order to develop a thermo-hydro-chemo-mechanical coupling model on fractured rocks. The THCM coupling model is compared with thermal, hydraulic, chemical, and mechanical coupling tests conducted at the University of Arizona. The comparison provides a reasonable prediction for the THCM coupling tests on various rock materials. Finally, the THCM coupling model for fractured rocks simulates the underground nuclear waste storage in Yucca Mountain, Nevada, and conducted performance and risk analysis on the repository.