Density spatial distribution and anisotropy of full-scale bentonite-sand blocks

Abstract

Constructing buffer barriers using high-quality bentonite-based blocks is crucial for the secure disposal of high-level radioactive waste (HLW). However, the density distribution and anisotropy of full-size bentonite-sand blocks are not yet fully understood, which may affect their quality and performance as buffer barriers. This study aimed to address this critical issue by conducting computed tomography (CT) scanning tests on full-size bentonite-sand blocks in the radial, angular, and vertical directions to obtain CT numbers and density. The analysis of the density distribution and anisotropy in these three directions followed. Subsequently, thermal conductivity and hydraulic conductivity tests were conducted in these directions to explore their relationship with density anisotropy. The findings revealed that the block density gradually decreased with increasing radius in the radial direction and first decreased slightly and then increased slightly in the angular and vertical directions. However, the density of the bentonite-sand mixture inside the blocks ranged from 1.96 g/cm³ to 2.09 g/cm³, with minimal differences. Additionally, there was no significant difference in density between the radial and angular directions, with an average density of 2.06 g/cm³. The density in the vertical direction was slightly lower, with an average density of 2.04 g/cm³. Scanning electron microscope images validated the presence of anisotropy in block density caused by horizontally oriented bentonite aggregates resulting from axial compression. However, the presence of quartz sand and incompressible impurities in the blocks created preferential flow paths, mitigating the impact of block density anisotropy on thermal and hydraulic conductivity. Consequently, thermal conductivity, averaging 1.60 W/m·K, and hydraulic conductivity, averaging 3.12 × 10−10 cm/s, demonstrated isotropy in all three directions. These block parameters meet the design requirements for an effective buffer barrier. This study systematically reveals the density distribution of full-size bentonite-sand blocks and its impact on the performance, providing important references for the safe design of HLW repositories.