Numerical simulation of long-term performance of deep geological repository placement rooms in crystalline and sedimentary rocks

Abstract

Evaluation of the stability of placement rooms and damages in the geosphere surrounding a deep geological repository (DGR) for high-level nuclear waste are essential components for assessing the overall repository performance. Assessments of the performance of placement rooms for the duration of the regulatory period (typically 1 million years) usually involves simulations using numerical codes capable of modeling 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 study, a novel approach for simulation of damage evolution and fracturing around the repository rooms, based on the bonded block model (BBM) devised for analyzing coupled THM processes, is used to perform a study of the long-term geomechanical performance of the Nuclear Waste Management Organization hypothetical DGR configurations in sedimentary and crystalline geological settings. The perturbation effects of different loading conditions, including in-situ stress changes by room excavation, heat generated from the used fuel canisters, and glaciation were analyzed. Each THM analysis was conducted using a 3D continuum numerical code to generate evolving temperature and pore pressure fields in the rock mass. The temperature and pore pressure data were then imported in the 2D BBM model for simulating evolution of damage around the placement rooms. Sensitivity analysis was conducted to investigate the effect of different poro-mechanical parameters, rock mass permeabilities, and different approaches for calibrating the BBM, on predictions of room stability and extent of the excavation damage zone around the placement room. The modeling results indicate that the confinement provided by the bentonite backfill inside the room clearly prevents any unravelling but also limits the extent of the damage to less than 1.6 m into the room walls in any of the analyzed cases considering different loading conditions and uncertainty of the material properties.