Advances in experimental investigation on hydraulic fracturing behavior of bentonite-based materials used for HLW disposal

Bentonite-based materials (BBM) in the form of blocks or rings have been commonly proposed as buffer materials for high-level radioactive waste disposal. In a real repository, hydraulic fracturing is likely to be present when the ground water pressure exceeds to hydraulic resistance, and the existence of technological gaps increases the risk of hydraulic fracturing. Based on the previous studies of hydraulic fracturing of BBM and technological interfaces, the present work systematically analyzes the experimental achievements. Results show that the fracturing pressure of BBM and interfaces increases with hydration time, initial dry density, bentonite content and overburden pressure, while it decreases with initial water content and gap thickness. The temperature effect on hydraulic fracturing depends on the predominant cation of bentonite. For Na-bentonite, the higher temperature leads to the larger swelling pressure, and thus the stronger hydraulic resistance. Tensile failure, shear failure and synthesis failures are increasingly accepted for hydraulic fracturing mechanism. The hydraulic fracturing of highly compacted BBM is mainly controlled by tensile failure, while the initial shear failure can be found for BBM with a low enough density. Since hydraulic fracturing is expected to occur several times during the repository operation, the long-time monitoring of hydro-mechanical behavior is of vital importance until the technological gaps are completely sealed. Furthermore, the failure mechanism of hydraulic fracturing of BBM should be further researched, as well as the sealing properties of interfaces under thermohydro-mechanically coupled conditions are proposed to be studied thoroughly.