Gas transport in a binary bentonite mixture used in sealing systems

Abstract

Binary mixtures composed of 80% of high-density pellets and 20% of granular bentonite at hygroscopic water content are currently considered for use in vertical sealing systems for the deep and longterm disposal of radioactive wastes. By pouring the dry components without compaction, the mixturedisplays a dry density of around 1.49 Mg/m3. However, these sealing systems are subjected to long-term hydrations from the surrounding formations with the possibility of having zones with entrapped and generated gas that makes it challenging to display a homogeneous saturation of the seal. Therefore, gas transport through these bentonite-based sealing materials at different degrees of saturation is a crucial issue. Hence, several oedometer tests have been conducted to evaluate the impact of gas injection on this mixture at different degrees of saturation and constant pouring dry density. These gas injection/dissipation tests were performed at constant vertical stress or constant volume conditions. The results provided the variation of the effective gas permeability at different degrees of saturation and the gas breakthrough pressure aftersaturation. Furthermore, these gas transport results were interpreted with a microstructural study of the mixture focusing on the volumetric filling of the inter-pellet porosity.