DEM-Based Methodology for Simulation of Long-Term Geomechanical Performance of a Placement Room in a Deep Geological Repository.

Evaluation of the stability of placement rooms and excavation damage in the surrounding geological formations of a deep geological repository (DGR) for high-level nuclear waste are essential components when assessing the overall repository performance. Assessment of the geomechanical performance of placement rooms for the duration of the regulatory period (typically 1 million years) usually involves simulations using numerical models capable of simulating coupled thermo-hydro-mechanical (THM) processes induced by different natural and repository-induced perturbations over different time and length scales. The numerical constitutive model used to represent the host medium should be capable of capturing highly nonlinear responses to stress changes, including development of damage and fracturing. In this paper, a novel approach for simulating the evolution of damage and fracturing around the repository rooms considering coupled THM processes is presented, including validation examples for the approach. The important and novel feature of this modeling approach is the use of the bonded-block model based on the distinct element method (DEM) to represent deformation, damage, and fracturing of the surrounding rock that could potentially result in rock fragmentation and collapse of the rock surrounding the placement rooms. As part of the study, the predictions of damage around placement rooms obtained using the new approach are compared with the conventional continuum mechanics model. Subsequently, the capability of the bonded-block model to simulate damage caused by in-situ and thermally induced stresses is validated by comparison with monitoring data and observations from field-scale experiments. Highlights: A novel numerical approach for modeling thermo-hydro-mechanical processes taking place around deep geological repositories is presented. The approach uses a coupling scheme that allows simulations of times relevant for performance assessment of the repositories. Formation of stress-induced microcracks and their effect on permeability changes and long-term stability of the rooms are explicitly simulated. The methodology is validated by comparison with field data and with a different numerical method. [ABSTRACT FROM AUTHOR]