Long-term modeling of the thermal-hydraulic-mechanical response of a generic salt repository for heat-generating nuclear waste

A modeling effort has been undertaken to investigate the long-term response of a generic salt repository for heat-generating nuclear waste, including processes that could affect the geological (natural salt host rock) and geotechnical (backfill) barriers. For this purpose, the TOUGH-FLAC sequential simulator for coupled thermal-hydraulic-mechanical processes modeling has recently been provided with a capability for large strains and creep. The responses of the saliferous host rock and the crushed salt backfill are modeled using dedicated constitutive relationships. Similarly, the coupling between the geomechanics and the flow sub-problems is performed on the basis of theoretical and experimental studies. The repository investigated in this work considers in-drift emplacement of the waste packages and subsequent backfill of the drifts with run-of-mine salt. Using the updated TOUGH-FLAC, the compaction of the backfill and the evolution of its properties as porosity decreases can be modeled. Additionally, different processes that may influence the initial tightness of the host rock can be investigated. On the basis of state-of-the-art phenomenological models, our simulation results show that, in order to evaluate the barriers integrity, it is necessary to consider full coupling between thermal, hydraulic and mechanical processes. A base case scenario that accounts for these coupled processes is presented and compared to a case in which the mechanical processes are disregarded. Also, we investigate the sensitivity of the coupled numerical predictions to two factors: the initial saturation within the host rock and the capillary forces. Although the outcome of these simulations is preliminary and will be improved as the understanding of relevant processes moves forward, the numerical tools required to perform the target predictions have been significantly improved.