Analytical modeling of bentonite swelling with experimental validations through hydrothermal reaction and free swelling index measurement

Bentonite has been widely used as buffer and backfill materials for nuclear waste disposal in last few decades. The main reasons of bentonite being selected for buffer and backfill materials include its low hydraulic permeability and high plasticity, which can restrict the transport of nuclear waste through the barrier and maintain the sealing ability of barrier, respectively, even with the deformation of bentonite. However, illitization process reduces the amount of montmorillonite in the buffer and its swelling ability, which may result in the loss of sealing ability and subsequent leakage of nuclear waste. This study presents an advanced analytical solution to describe the movement of swelling clay after absorbing water, combined with Darcy's law and constitutive laws of stress and strain with the consideration of changing permeability and porosity with time. The developed analytical solution has been validated with experiment results of two different types of bentonites-Gyeongju bentonite and Bentonil. Free swelling index with different montmorillonite/illite fractions were measured and compared with the analytical solution, as showing the average errors of lower than 5%.