The role of coupled hydro-chemo-mechanical modelling in the optimisation of a repository closure system

The closure of Cigéo, the future deep geological disposal facility for radioactive waste in France, will rely on the use of swelling clay-based seals. Their main safety functions depend on the development and stability of swelling pressure that is transferred to the surrounding materials and host rock. Under such conditions, the seal has low permeability to water, while the connected fractured zone in the host rock is under compression. If bentonite-sand mixtures are used, gas permeability can be engineered so that gas pressure build-up due to hydrogen generation in the repository remains below pre-defined threshold values. It is clear that the design of such closure systems need not only to consider engineering aspects of construction and short-term effects, such as tunnel convergence during operation, concrete liner loading, or bentonite swelling upon hydration. Long-term effects become a key aspect, such as the effect of gas generation and pressure build-up on the long-term saturation of the seal, the geochemical interaction between different barriers and the host rock, which can impair the longevity of the engineered barriers, or long-term creep effects in concrete and host-rock.

In this contribution, we present a methodology that is developed as an informative tool for optimizing different closure systems while considering long-term performance as a central aspect. A numerical modelling framework that accounts for the governing complex physical and chemical processes involving hydro-chemo-mechanical couplings, which can result in changes in the swelling pressure, has been implemented in iCP, an interface between Comsol Multiphysics and Phreeqc. With this framework, different engineering designs and alternative materials of the sealing systems for horizontal, decline, and shaft seals have been simulated, considering their evolution over long time scales, until up to 100,000 years. The results are used as an informative tool for decision making, with long-term performance as a key aspect of the engineered solution.

These models encompass fluid flow under partially saturated conditions, nonlinear solid mechanics, and especially coupling with realistic geochemical models of the key reactive transport processes. Special emphasis on the coupling and interfacial processes between the seal and the surrounding materials (concrete supports, concrete liner, compressible layer, and the surrounding Callovo-Oxfordian claystone) is placed. The effects of the geochemical evolution of bentonite on its swelling pressure at the seal scale are quantified. The mechanical response of the system obeys not only to the hydro-mechanical couplings, but also to the impact of geochemical interaction of the bentonite-based seal with surrounding materials.

We focus here on the setup of the framework and an overview of the outcomes when applied to several alternative designs and scenarios. Long-term performance assessment covers the post-closure period until up to 100,000 years. Furthermore, a set of sensitivity analyses is conducted to quantify the uncertainty of several modelling assumptions and simplifications.