Dynamics of large-scale THM simulations: numerical considerations for the simulation of nuclear waste repositories in rock salt during glacial cycles

This study addresses the coupled thermo-hydro-mechanical (THM) modelling of large-scale nuclear waste repositories situated in rock salt. We consider the competing stability requirements arising from different physical processes, such as advection and diffusion in the rock mass surrounding the saline barrier, as well as viscoplastic creep of the latter. Our focus extends to examining potential conflicts between spatial and temporal discretization demands, balancing performance, stability and accuracy. We systematically investigate the applicability of these considerations to a model implemented in OpenGeoSys, aiming to enhance its robustness. An essential aspect of evaluating simulation results is analyzing the convergence behavior in the absence of an analytical solution. Several methods exist to extrapolate model results of different discretization to a reference set, such as the Richardson extrapolation. Here, we use such a technique to scrutinize our results, providing a comprehensive assessment of the model's convergence characteristics and the accuracy of the results. Finally, detailed comparisons across various coupling schemes are conducted for our model, highlighting the impact of implicit process coupling on the model results.