Combined experimental and numerical study on the fracture pattern of the system host rock and engineered barrier in deep geological repositories

In the safety assessment of high-level nuclear waste geological repositories, the stability of placement caverns and boreholes is a critical factor. However, rock masses are exposed to complicated mechanical, thermal, and hydraulic loading conditions, which may induce the propagation of pre-existing fractures or the initiation of new fracture networks. Fracture propagation not only compromises the structural integrity of boreholes but also provides pathways for radionuclide leakage. Excavation, thermal effects from nuclear waste emplacement, and backfill sealing may all induce fracture development. In many disposal concepts, granite may serve as host rock, whereas the canisters containing the nuclear waste are embedded in compacted bentonite.

Therefore, the thermal-hydro-mechanical behavior of the system host rock (granite), engineered barrier (compressed bentonite) and nuclear waste container must be described from the excavation during the emplacement of the nuclear waste canister until encapsulation. In this study, a procedure is presented, starting from experimental studies of the fracture behavior of the granite and compressed bentonite, leading to numerical modeling with the Finite-Discrete Element (FDEM) Code Irazu (Geomechanica) to simulate fracturing and deformation patterns under various in-situ stress fields and temperature conditions.

In the laboratory experiments, the Semi-Circular Bend Test (SCB) and Double-Edge Notched Brazilian Disk Test (DNBD) are conducted to measure the fracture toughness of bentonite and granite under ambient conditions and heat treatments. Optical microscopic observation is conducted on thin slices to evaluate the thermal effects on the microscopic structure of granite, and the thermal damage mechanism is further analyzed. High-speed cameras and digital image correlation (DIC) technique are used to trace the fracture process, and important fracture parameters, such as the fracture initiation, fracture process zone (FPZ) size and critical opening displacement, are measured by full-field displacement and strain measurement.

Based on FDEM, a numerical model corresponding to laboratory tests is developed to simulate the entire process of fracture initiation, propagation, and ultimate failure. The simulation results are compared with experimental observations obtained using the DIC technique. Model parameters are calibrated against experimental data to construct a robust numerical model, which is then employed to simulate fracture behavior during distinct phases of a nuclear waste repository: the excavation stage, the thermal effect stage induced by the placement of nuclear waste canisters, and the backfill sealing stage. This approach provides theoretical support for investigating fracture behavior under repository excavation, operation and closure scenarios. Also, hydraulic parameters are to be derived from the various scenarios, helping to reveal the evolution patterns and influencing factors of permeability coefficients and to optimize the cavern and borehole design.

Keywords: Deep geological repositories; Host rock and engineered barrier; Fracture pattern; FDEM; Thermal treatment