Deterioration of exposed buffer block: desiccation shrinkage and cracking

In high-level radioactive waste (HLW) repositories, buffer blocks are used for constructing engineering barrier to intercept radionuclides. Deterioration of exposed buffer blocks during their manufacture, storage, and transport may enlarge the hydraulic conductivity of an engineering barrier, reducing the barrier’s efficacy. It is not easy to monitor the shrinkage displacement by traditional tools because the shrinkage is less than 1 mm per day, while the monitoring length of the block is as large as 350 mm. Additionally, the cracks are often too narrow to allow for an accurate measurement of their depths. For this study, we used Gaomiaozi (GMZ) bentonite and quartz sand at a 7:3 mass ratio to compact fan-shape buffer blocks, and then exposed those blocks in an indoor environmental condition, which allowed desiccation of the blocks to occur. We monitored the overall and surface shrinkage of the blocks employing fixed dial indicators (FDIs) and digital image correlation (DIC), respectively. We excavated subsamples containing cracks from a desiccated block at various depths and estimated the depth of the cracks using Brazilian split tests. We also investigated the microstructure evolution of the blocks with mercury intrusion porosimetry (MIP) after exposure. Hierarchical evaporation of the block’s moisture was observed because the evaporating surface migrated inward rapidly, which led to the absence of initial evaporation stage. The shrinkage displacements of each measuring line were similar, while the longer measuring lines presented less shrinkage strain, indicating the uneven shrinkage. Moreover, the surface shrinkage of the blocks was greater than the overall shrinkage, revealing the interior of the blocks to be resistant to change during desiccation. Besides, the blocks’ inner pores showed less change, while the inter-aggregate pores on the block surface shrank visibly after exposure. The depth of the desiccated cracks was less than 26 mm, as estimated by the distribution of tensile strength. Because we found the varied deterioration levels between the block’s surface and interior, we determined that its structural integrity and efficacy were not as compromised as its surface seemed to indicate.