Multiphysical Testing and Computational Modeling of Microfiber-Reinforced Bentonite Clay for Geological Repositories of Nuclear Spent Fuel
Authors: Azzam Abdullah , Rahmani Mohammad , Kim Yong-Rak , Eun Jongwan , Kim Seunghee 
Published: August 2024

ABSTRACT:
The increase in nuclear waste worldwide and the need to provide a secure storage and disposal method has raised a great concern to protect public health, safety, and energy security. Disposing of the waste deep underground in the geological repository is the most preferred long-term solution for nuclear waste disposal. The canister containing the high-level waste is buried deep underground and encapsulated by engineered barrier material (EBM), which separates it from the natural rock. Bentonite clay is the most preferred EBM due to its advantageous properties, such as low cost, high thermal resistance, and low permeability. However, bentonite is subjected to a complex multiphysics environment as the canister generates heat, causing bentonite to desiccate, while moisture infiltrating from the host rock causes bentonite to swell. A better-performing EBM that is less permeable and more resistant to desiccation cracking and chemical degradation is beneficial, and an inorganic microfiber-reinforced EBM (i.e., IMEBM) can be a promising solution to the challenge. This study investigates the effects of carbon microfibers embedded in bentonite on developing such IMEBMs. Experimentally, a 3-D soil tube test was conducted incorporating moisture (mass) and heat transfer. This test aims to identify multiphysical material characteristics influenced by adding microfibers in bentonite. The test conducted with pure bentonite was then modeled using a multiphysical finite element (FE) method to predict temperature distribution and flow rate of moisture through the porous domain that is a function of geologic repository conditions: moisture and temperature. Test results indicate that the temperature developed in the IMEBM was 20% lower compared to the plain bentonite case, which resulted in lesser moisture loss compared to plain bentonite. This in turn altered desiccation cracking behavior. Microfiber effectively maintained moisture and reduced major desiccation cracks through a "crack bridging" effect.

1. INTRODUCTION

1.1 Research Significance
Desiccation cracking is a widely occurring phenomenon in soil that develops as soil loses moisture during the drying process which affects its mechanical and hydraulic properties (Corte & Higashi, 1964; An et al., 2018; Nahlawi et al., 2004; Tollenaar et al., 2017). Desiccation of soil causes significant problems in geological applications such as engineered barrier system (EBS) in deep geological repositories (Miller et al., 1998; Omidi et al., 1996, Rayhani et al., 2007). Nuclear waste canisters buried deep underground (500 to 100 meters) generate high temperatures (Johnson et al., 2002) that cause surrounding buffer material (commonly bentonite clay) to shrink and desiccate (Peron et al., 2009). Due to the mechanically restrained and confining configuration of EBS, tensile stress builds up, and upon exceeding the material limits desiccation cracks initiate and propagate (Nahlawi et al., 2004; Zornberg et al., 2007). However, surrounding host rocks provide moisture to EBM causing buffer material to swell and heal. This multiphysics environment is known as a thermo-hydro-mechanical (THM) environment.