High-Temperature Performance of Compacted Bentonite Blocks for High-Level Nuclear Waste Repository: A Study of Desiccation Crack Control and Gas Permeability with Glass Microfiber Reinforcement
Authors: Noh D.-H. , Kim S. , Eun J. , Kim Y.-R. 
Published: August 2024

ABSTRACT:
Geological repositories for high-level nuclear waste require barriers with low permeability to prevent radionuclide migration. Compacted bentonite, which exhibits low permeability and high swelling capacity, is considered an ideal candidate for such applications. However, the thermal conditions within repositories, potentially reaching temperatures of 200°C, pose a risk of moisture loss and desiccation cracking, which can compromise the sealing performance of bentonite. This study investigates the use of glass microfiber reinforcement to enhance the structural stability of bentonite blocks against thermal challenges in geological repositories. We investigated the gas permeability and crack formation in bentonite blocks reinforced with varying concentrations of glass microfiber. The results indicated that the fiber content of C = 1% effectively reduces crack size without altering the gas permeability, maintaining the sealing performance close to that of pure bentonite. However, a higher value of the gas permeability was found in the bentonite block with the fiber content at C = 2%. X-ray CT imaging provided insights into the microstructural changes, confirming the role of the glass fiber in mitigating crack formation. This study underscores the importance of optimizing fiber content in bentonite-based barriers to ensure the good sealing performance of waste containment systems under high thermal conditions.

1. INTRODUCTION
Deep geological repositories use highly compacted bentonite blocks to seal radioactive waste, thereby preventing leachate from the radioactive waste into the surrounding environment. As such, thermo-hydro-mechanically coupled behaviors of bentonite blocks have been intensely studied to enhance their performance for the deep geological repository of high-level nuclear wastes (Smith et al., 1980; Yong et al., 1986; Cho et al., 1999; Cho et al., 2000; Lloret et al., 2003). In the geological repository setting, bentonite would surround the radioactive waste canister in the form of a compacted block with a high dry density. This bentonite block may encounter high temperatures as high as 200°C, based on factors such as waste heat emission, spacing of waste packages, and the thermal conductivity of the bentonite blocks and host rock (Martin et al., 2014; Abootalebi and Siemens, 2018). When the temperature of the repository exceeds 100°C, the bentonite buffer may lose moisture in its pores. As a result, not only does the swelling pressure reduce, but desiccation cracks also form due to the shrinkage in the volume (Villar and Lloret, 2004; Gebrenegus et al., 2011; Ye et al., 2013; Sánchez et al., 2013; Kim et al., 2021). The reduction in the swelling pressure and the presence of such desiccation cracks could compromise the desired sealing performance of the bentonite block (Pusch, 1980; Gens and Garcia-Fontanet, 1997; Martin et al., 2006; Åkesson et al., 2009; Chang et al., 2022; Gens et al., 2022;). In this regard, the addition of additives such as graphite aggregates, sands, copper wires or powder, and inorganic microfibers have been examined by several research groups to improve the sealing performance of the bentonite buffer, mainly by increasing the thermal conductivity or imparting the tensile stress between bentonite particles (Liu et al., 2019; Wang and Hadgu, 2020; Peng et al., 2021; Feng et al., 2024). On the other hand, adding those additives brought up a new concern that the inclusion at high percentages could potentially compromise the hydraulic sealing capacity of bentonite blocks because it may lead to predominant fluid flow pathways. Therefore, further in-depth study is needed to examine the optimal contents of such additives that can help maintain the sealing integrity while improving the mechanical performance of bentonite blocks.