In the French CIGEO concept, bentonite-based materials will be used to seal access facilities such as galleries and shafts. Their primary role is to create a low-permeability barrier that limits advective water flow, thereby delaying the migration of radionuclides to the biosphere. In addition, these bentonite-based seals are designed to fulfil an additional safety function by allowing gas to migrate, thereby preserving the favourable confinement properties of the host rock. Meeting these requirements implies that the bentonite sealing systems must combine two seemingly contradictory properties: high gas permeability and low water permeability. To achieve this, the use of mixtures with high sand content has recently been considered, but this introduces significant challenges. While sand improves structural strength, reducing the bentonite content can compromise sealing effectiveness. This issue becomes more critical in the presence of alkaline plumes arising from interactions with concrete structures, which can lead to smectite alteration and sand dissolution. These processes modify the microstructure, increase hydraulic conductivity, and further reduce the mixture’s sealing capacity.
This post-doctoral project, carried out at ASNR within the framework of the European Joint Programme EURAD-2 and the ANCHORS project, aims to investigate the behaviour of bentonite–sand mixtures with high sand contents under coupled hydraulic, chemical, and gas loadings across multiple scales.
The project is particularly focused on improving the understanding of physico-chemical phenomena to support the development of coupled modelling approaches relevant to the safety assessment of future geological disposal facilities. The reference material will consist of MX80 bentonite–sand mixtures (40/60 dry mass ratio), extended to higher sand contents and different sand characteristics (e.g. grain shape and granulometry). The work will combine advanced coupled HM experiments under controlled hydraulic and chemical boundary conditions, including in situ micro-CT investigations using PMMA cell prototypes, together with state-of-the-art microstructural and mineralogical characterization techniques. These tests will enable the investigation of gas migration mechanisms as well as the impact of chemical perturbations on the swelling behaviour and hydraulic performance of the tested mixtures. The macroscopic investigations will be complemented by microstructural and mineralogical analyses using techniques such as X-ray diffraction (XRD), scanning electron microscopy coupled with EDS (SEM/EDS), and BET surface area measurements. These approaches will provide detailed insight into mineral alteration processes and pore structure evolution during hydration and gas transfer. The project will also include the development of advanced image analysis tools for the segmentation of low-contrast phases (e.g. saturated/desaturated porosity), based on existing methodologies and/or machine-learning approaches.
Considering the long duration and complexity of the planned experimental programme, a duration of 24 months is considered necessary to ensure the scientific robustness and completeness of the project. The coupled hydration, chemical perturbation, and gas injection tests on bentonite–sand mixtures require long equilibration and monitoring periods, with each experiment potentially extending over several months depending on the imposed boundary conditions and the targeted hydro-mechanical state. In addition, the project involves the investigation of several mixture configurations with different sand contents, dry densities, and sand characteristics (e.g. granulometry and grain shape), significantly increasing the number of experimental conditions to be explored. Furthermore, the interpretation of the coupled processes relies on the integration of macroscopic observations with detailed microstructural and mineralogical analyses performed before and after testing. The implementation of in situ micro-CT monitoring and advanced image analysis approaches also requires dedicated calibration and optimisation, phases before systematic testing can be performed.
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