Hydro-thermal impacts on near-field flow and transport in a shale-hosted nuclear waste repository

Abstract

Performance of geologic radioactive waste repositories depends on near-field and far-field processes, including km-scale flow and transport in engineered and natural barriers, that may require simulations of up to 1 M years of regulatory period. For a relatively short time span (less than 1000 years), the coupled thermo-hydro-mechanical–chemical (THMC) processes caused by heat from the waste package will influence near-field multiphase flow, chemical/reactive transport, and mechanical behaviors in the repository system. This study integrates the heat-driven perturbations in THM characteristics into THC simulations using PFLOTRAN to reduce dimensionality and improve computational efficiency by implementing functions of stress-dependent permeability and saturation-temperature-dependent thermal conductivity. Our reduced-order THMC simulation of a single waste package in a shale-hosted repository reveals three physical mechanisms associated with heat pulse emitted from the waste package: (1) increase of liquid mobility that accelerates the inflow and solute transport from the fully saturated host rock, (2) production of thermal gradients that results in outward flow within the buffer and disturbed rock zone (DRZ) nearest the waste package, and (3) reduction of thermal conductivity that confines heat within the engineered barrier system (EBS). These combined hydro-thermal mechanisms influence the rate of re-saturation process and transport/exchange of corrosive species in the repository system.