Thermo-hydro-mechanical properties of bentonite-sand-graphite-polypropylene fiber mixtures as buffer materials for a high-level radioactive waste repository

Abstract

In accordance with the conception of repository, the performance of buffer materials around the canister are critical to the long-term safe operation of deep geological repositories for high-level radioactive waste (HLW). To investigate the thermo-hydro-mechanical (THM) properties of bentonite-sand-graphite-polypropylene fiber (BSGF) mixtures as buffer material, the thermal and hydraulic conductivity, swelling and cracking, strength tests were conducted in this paper. From the microscopic, the heat transfer mechanism, penetration mechanism, swelling mechanism and shear mechanism of BSGF mixtures were investigated in detail. Results revealed that the addition of graphite and polypropylene fiber (PPF) can significantly improve thermal conductivity and strength of BSGF mixtures. Compared with bentonite-sand mixtures, the thermal conductivity, the unconfined compressive strength (UCS) and shear strength (cohesion) of BSGF mixtures increased by 20–68%, 12–23% and 21–48%, respectively. Nevertheless, the internal friction angle had little change. The addition of PPF controlled the width, area, and propagation of the cracks through the interlacing effect of the fibers. However, the addition of graphite and PPF can also hinder the development of swelling pressure and improve the hydraulic conductivity of BSGF mixtures. In addition, for BSGF mixtures used as buffer material of HLW storage, the sand content should be retained below 31% and 32% respectively to ensure the maximum swelling pressure and hydraulic conductivity to meet the requirements of the specification. From the view of heat dissipation and strength, sand content shall not exceed 30% and 27%, respectively. Therefore, the effective sand content should not exceed 27% considering the THM properties. This study provides a new buffer material for HLW deep geological repository and enhances the stability and functionality of HLW deep geological repository.