Effect of relative humidity on the desiccation behavior of bentonite buffer blocks for geological repositories

Abstract

This study evaluated the hygrothermal desiccation behavior of compacted sodium bentonite buffer blocks, which are critical components of Engineered Barrier Systems (EBS) for high-level radioactive waste repositories. To address the risks associated with moisture loss during storage and installation, a coupled moisture-transport–shrinkage model incorporating capillary liquid flow, vapor diffusion, and volumetric deformation was developed and validated against controlled environmental chamber tests. The model utilized vapor–liquid equilibrium principles to quantify the influence of pore capillary structure on water vapor thermodynamic activity, establishing a consistent theoretical link between relative humidity (RH) and water saturation. Experimental results indicated that blocks compacted to a dry density of 1.61 g/cm 3 with lower initial saturation ( S e = 80%) exhibited reduced sensitivity to RH fluctuations compared to fully saturated blocks. For this tested condition, volumetric shrinkage of up to approximately 9% was observed under dry conditions; volume reduction stabilized when ambient RH fell below 45%. To mitigate shrinkage and preserve the essential thermal interface with waste canisters, the study recommended maintaining an environmental RH above 75% during operational phases. These findings provide a physically based framework for assessing environmental effects on bentonite barriers, and offer guidance to support design and handling strategies for geological repositories.