Reduced-Order THMC Coupled Simulation of Nuclear Waste Disposal in Shale

Abstract

ABSTRACT: Thermal and hydrological behaviors of multiphase pore fluids in the presence of heat cause the near-field thermo-hydro-mechanical-chemical (THMC) coupled processes that can influence performance of geologic radioactive waste repositories. This hydro-thermal impacts may perturb the geomechanical stability of the disturbed rock zone (DRZ) surrounding the drifts in a shale-hosted deep geologic repository, which links heat/fluid flow and chemical/reactive transport between the engineered barrier system (EBS) and the host rock. This work focuses on integrating the effects of a near-field geomechanical process driven by buffer swelling into TH simulations to reduce dimensionality and improve computational efficiency. This geomechanical process can reduce the DRZ permeability, potentially influencing the rate of radionuclide transport and exchange with corrosive species in host rock groundwater that could accelerate waste package degradation. The sensitivity test with variation in host rock permeability indicates that less permeable shale retards re-saturation of the buffer, such that slower increase of swelling pressure delays reduction of DRZ permeability. 1. INTRODUCTION Clay-rich sedimentary strata (shales and argillaceous formations) are one of the potential geologic media for disposal of radioactive waste in the United States [Shurr (1977), Gonzales & Johnson (1985)] due to their low permeability, high sorption capacity, typically reducing porewaters (which limit radionuclide solubility), and ability to deform elastically/plastically, which promotes self-healing of fractures [Neuzil (2013)]. However, clay-rich rocks tend to be less conductive thermally due to mineral composition [Schon (2011)], which implies that the evolution of shale-based repository systems may be more sensitive to thermal loading/unloading. In the near-field, the buffer and the host rock mainly influence each other thermally by heat flow, hydraulically by single- or multi-phase fluid flow, mechanically when the buffer absorbs water from the rock and swells, and also chemically by exchange of solutes between groundwater and pore water in the buffer. The changes in hydrological, mechanical and geochemical characteristics of the disturbed rock zone (DRZ) will affect the overall behavior of radioactive waste geological repositories by redistributing stress states, creating additional pathways of solute/radioactive transport or fluid flow [Tsang et al. (2005)]. These simultaneous actions will induce a number of Thermo-Hydro-Mechanical and Chemical (THMC) processes that will significantly affect the evolution and long-term performance of the nuclear waste repository system [Bernier et al. (2017)].