Thermomechanical Analysis of Fiber–Bentonite-Based Mixtures as Buffer Material in an Engineered Nuclear Barrier

Abstract

Buffer/backfill material is an important part of engineered barriers in deep geological repositories of high-level radioactive waste (HLRW). A good buffer material not only possesses good heat conductivity, expansion, and sealing characteristics, but also exhibits enough strength. Its thermomechanical performance can significantly influence the safety and stability of a HLRW repository. Polypropylene fibers were mixed with bentonite-based mixtures (bentonite–sand–graphite) to form a buffer material with high strength, good heat dissipation capacity, and strong isolation capacities. The effects of fiber content, fiber length, and sand particle size on the shear strength, swelling pressure, and thermal conductivity of fiber–bentonite-based (BSGF) mixtures were analyzed under given graphite and sand contents. Detailed analyses of the shear, expansion, and heat transfer mechanisms of BSGF mixtures were performed to understand the behavior. The results indicated that the strength of BSGF material improved significantly with the addition of fiber content, whereas swelling potential and thermal conductivity were reduced. With an increase in sand particle size, cohesion and maximum swelling pressure decreased, whereas internal friction angle and thermal conductivity increased. The optimum fiber content, fiber length, and sand particle size were determined based on the thermomechanical performance of BSGF mixtures. These findings help in the field application of buffer materials with different fiber contents, fiber lengths, and sand particle sizes.