Experimental study on boundary-conditioned hydration impact on bentonite gas permeability with supporting modeling

Abstract

Deep geological disposal is currently a widely accepted disposal option for high-level radioactive waste, with bentonite serving as a crucial buffer/backfill material due to its self-sealing properties. Upon groundwater infiltration, the bentonite experiences constrained swelling due to surrounding rock confinement, influencing its hydro-mechanical behavior. While extensive research has explored the relationship between hydration and gas permeability under free-swelling conditions, the impact of constrained swelling remains insufficiently understood. In the present study, these properties under both free and constrained swelling conditions are investigated. The findings reveal that constrained swelling conditions hinder the hydration process, leading to reduced porosity and increased water saturation compared to free-swelling conditions. The temporal variation in water content under both boundary conditions can be described by an adsorption-diffusion equation with a strong correlation (coefficients of determination, R² > 0.97, and root mean squared error (RMSE) ≤ 0.0048). Moreover, the van Genuchten (VG) model accurately captures water saturation trends with R² = 0.99 and RMSE = 0.012 for free swelling, and R² = 0.98 and RMSE = 0.013 for constrained swelling). Despite having comparable water content, constrained swelling significantly reduces gas permeability. Additionally, a higher initial equilibrium relative humidity (RH) corresponds to a larger subsequent gas permeability gap, reaching 2.01 × 10⁻¹⁶ m² at 75 % RH and 5.72 × 10⁻¹⁶ m² at 98 % RH. Void ratio and water saturation, two critical parameters influencing gas permeability, undergo substantial changes during the hydration process. Specifically, changes in the available void ratio due to hydration affect gas permeability. By quantifying post-hydration equilibrium changes in void ratio and saturation, a gas permeability model accounting for different boundary conditions is developed, which demonstrates a relatively effective agreement with experimental data (R² = 0.75). This study addresses the knowledge gap on how boundary effects of bentonite hydration influence gas permeability, offering key insights into its long-term performance as a barrier material in nuclear waste repositories.