Thermo-hydro-mechanical calibration modelling of the full-scale emplacement experiment and sensitivity analyses

Abstract

To understand the mechanism of the coupled thermo-hydro-mechanical (THM) processes in a repository, many laboratory or in-situ experiments have been carried out. One of these experiments is the Full-scale Emplacement Experiment (FE-Experiment) at the Mont Terri Underground Rock Laboratory. The FE-Experiment simulated the construction, waste emplacement, backfilling and early post-closure evolution of a spent fuel/vitrified high-level waste disposal tunnel according to the Swiss repository concept. The FE tunnel was excavated between November 2010 and May 2011, and following a ventilation period, three heaters (simulant waste canisters) were installed in the FE tunnel during 2014. The heaters sat on pedestals made of bentonite blocks and the remainder of the tunnel was filled with a granular bentonite mixture (GBM). In this paper, a fully coupled THM COMSOL model was built and used to calibrate the thermal, hydraulic and mechanical parameters based on the measurements in the FE-Experiment. A set of thermal, hydraulic and mechanical parameters was obtained through this calibration and the mechanism of the coupled processes in the FE-Experiment was explained using this model. The couped THM model was also used to perform sensitivity studies. A sensitivity study examined the influence of assumptions regarding the heater representation in the model (geometry, thermal properties, application of the heat load) and proved that these assumptions had a significant influence on the heater temperatures, the temperature of the placement tunnel sealing material but not in the Opalinus clay (OPA) rock surrounding the placement tunnel. A second sensitivity study proved that the initial pore water pressure in the Opalinus clay had a significant influence on the pressure accumulation over time. A third study confirmed that using as-placed (unsaturated) thermal conductivity for the bentonite sealing material can give conservative thermal results.