This project is conducted by IRSN in the frame of research in connection with the Cigéo (project for a deep geological disposal facility for HA/MAVL radioactive waste). It is particularly aimed at understanding and modelling of physico-chemical phenomena that are important for the safety of the future facility. As a significant production of hydrogen is expected inside the geological repository, this gaseous phase could modify the flows and mechanical conditions of the rock and of the engineered materials such as bentonite barriers and sealings and therefore potentially impact the transport of radionuclides.

Bentonite-sand mixtures are considered potential sealing materials. The performance of these mixtures depends strongly on the sand-to-bentonite ratio: increasing the sand content can reduce swelling capacity and have an impact on water and gas permeabilities, particularly at lower bentonite content or dry density.

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 from concrete degradation, 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.

The work proposed in this post-doctoral fellowship as part of EURAD 2 (a european partnership) consists mainly in working on the characterization of the microstructure of Bentonite sand mixture under hydraulic, gas and chemical loadings using X-ray microtomography (X-ray micro-CT).

This work involves the use of PMMA cell prototypes to analyze the behavior of compacted mixtures. Gas injection tests will be conducted at constant volume conditions, with the mixture hydrated using a hyperalkaline solution. Microstructural analysis will be performed at the initial state, during hydration, and throughout gas injection. Regular X-ray micro-CT scanning will track the evolution of macropores, heterogeneous distribution zones, and pathways that may form due to the partial void filling by bentonite between sand particles. To optimize experimental conditions, the existing PMMA cell prototype may be upgraded, for instance, by integrating additional sensors.

Finally, an important part of the work will be related to the image analysis tools to improve the segmentation of low contrast grey level areas (extraction of saturated and desaturated pores). It will be possible to use the 2D tools developed at LETIS (texture analysis, random walkers and 3D active contour methods) or to start a new approach based on neural networks (CNN/DL). The methods set up should eventually become available to the scientific community in the form of Fiji software plug-ins.

PhD in Civil, geotechnical or geological engineering,
Proficiency in X-ray microtomography and Digital Image Correlation is required.

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