Analysis of the THM behaviour in a clay-based engineered barrier system (EBS): modelling of the HE-E experiment (Mont Terri URL)

Abstract

To understand the complex thermo-hydro-mechanical (THM) processes and their evolution in a clay-based engineered barrier systems (EBS) during the closure phase of a geological repository for radioactive waste, a 1:2 scale in situ heating experiment (HE-E experiment) has been ongoing since 2011 in the Mont Terri Rock laboratory. Based on the experimental data, a fully coupled THM 3D simulation using the finite-element program OpenGeoSys was carried out. The main objectives of the simulation were to interpret the experimental observations, to understand the thermally induced THM interactions and to analyse the different material properties. Five experimental phases were numerically interpreted, in order to take into account the changes in the temporary material properties during experimental operations. These included the tunnel excavation, the ventilation, the emplacement of the bentonite EBS, the heating phase and the phase after shutdown of the heater. A non-isothermal Richards’ flow model was used to take into account the evaporation and vapour diffusion during the heating. The material behaviours of EBS in association with the saturation-dependent thermal conductivity and the water retention behaviour under high temperatures were analysed in detail. The strong thermal, hydraulic and mechanical anisotropic properties of the Opalinus Clay were described by a transversely isotropic model.