Hydro-mechanical study of MAVL nuclear waste repository galleries in Callovo-Oxfordian claystone

The disposal of radioactive waste in deep underground repositories has been studied for a long while (OECD/NEA, 1995). Argillaceous rocks have been found to be good candidates to host the repositories because of their low permeability and the ability to absorb radionuclides. In France, the Callovo-Oxfordian argillite (COX) has been chosen by the national agency ANDRA to be the host rock to store the radioactive waste.

During the previous years, the problem of gallery excavations in the COX host rock has been the case of study in Liège University. The problem, involving strain localization, is not well posed when modelled using classical theories; a microstructured model is proposed: local Second Gradient (Collin et al. 2006), this avoids the pathological mesh dependency by introducing an internal length that regularizes the problem.

At the present time, the model takes into account several transverse anisotropies, e.g. cohesion (Pietruszczak et al. 2002), a visco-plastic model is retained to model the long term convergence, finally, a permeability evolution model allows to correlate strain localization and permeability increase (Pardoen et al. 2016).

The purpose of this work is to model the long term behaviour of the MAVL galleries (100 years) with a special focus on the localisation extent, effect of ventilation, displacements and concrete stress state. Results (Figure 1) show an important effect of the supporting structure on the problem, the compressible wedges determine the localization mode by triggering the shear bands. Results also show a high sensitivity to the viscosity parameters.