Modelling gas diffusion in compacted water-saturated Na-bentonite considering multi-porosity effects

Predicting the gas diffusion coefficient of water-saturated Na-bentonite is crucial for the overall performance of the geological repository for isolating high-level radioactive waste (HLW). In this study, a conceptual model that incorporates a multi-porosity system was proposed, dividing the pore space into free water pores, interlayer water pores, and diffuse double layer (DDL) water pores, to describe the molecular diffusion behaviour of the dissolved gas in saturated bentonite. In this model, gas diffusion in these three porosities is considered as independent and parallel processes. The apparent gas diffusion coefficient is quantified by applying weighted approximations that consider the specific porosity, tortuosity factor, and constrictivity factor within each porosity domain. For verification, experimental data from gas diffusion tests on saturated MX-80 and Kunipia-F bentonite specimens across a wide range of dry densities were utilized. The proposed model could successfully capture the overall trend of the apparent gas diffusion coefficient for bentonite materials across the partial dry density of montmorillonite ranging from 900 kg/m 3 to 1820 kg/m 3 , by employing only one fitting parameter of the scaling factor. When the partial dry density of montmorillonite decreased to 800 kg/m 3 , the proposed model shows an underestimation of the apparent gas diffusion coefficient due to possible changes of the tortuosity factor. Model predictions indicate that gas diffusion in saturated bentonite is primarily controlled by the free pore domain, with minimal contributions from DDL pores. Despite being the dominant pore type, interlayer pores contribute limitedly to total D a / D w values due to significant constrictivity effects.