Computational-Experimental Approach for Determining Material Parameters of Bentonite Clay in Engineered Barrier Systems for Nuclear Waste Repositories

Abstract

ABSTRACT Bentonite clay is a common material used in engineered barrier systems (EBS) to isolate the nuclear waste and protect the environment. However, its performance can be affected by changes in environmental conditions, such as high temperatures and water evaporation. The high temperature around the canisters causes the water in the bentonite to evaporate, leading to shrinkage and desiccation cracking, which in turn may compromise its sealing function and increase the risk of contaminating the surrounding soil. This study aims to determine the shrinkage and fracture properties of desiccating bentonite clay. To accurately understand and predict the desiccation behavior of the engineered buffering material, restrained ring tests were conducted and the shrinkage behavior was observed using a digital image correlation system. A finite element model was used to simulate shrinkage and desiccation cracking of bentonite clay. Through an inverse optimization process, the shrinkage coefficients and moisture-dependent cohesive zone fracture parameters of the bentonite were inversely identified. These fracture and geometrical stability parameters are essential for designing effective engineered barrier systems for the safe storage of nuclear waste. The presented experimental-computational method can determine material characteristics subjected to complex multi-physical conditions with damage. INTRODUCTION The potential risk associated with desiccation cracking exists in many engineering applications, such as designing engineered barrier systems (EBS) for geological repositories (Narani et al., 2020; Rayhani et al., 2007). When exposed to high temperatures, water evaporation within the buffer material [commonly bentonite clay] adjacent to the nuclear-spent fuel canisters leads to shrinkage of the bentonite body (Peron et al., 2009). Considering the confined configuration of EBS, the induced shrinkage process becomes mechanically restrained, leading to the development of tensile stresses and desiccation cracking (Nahlawi et al., 2004). As the soil desiccates, its stiffness, tensile strength, and properties of its bonding with other materials can significantly change (Sanchez et al., 2013; Song & Cui, 2020; R. Tollenaar et al., 2017). Understanding the evolution of the buffer material properties throughout the lifetime of a geological repository is imperative, as these systems are designed to provide long-term confinement and isolation of nuclear-spent fuels.