Carbonate-Induced Self-Sealing of Near-Field Granite Fractures in Geological Disposal of High-Level Radioactive Waste: Coupled THMC Precipitation–Dissolution Mechanisms and Long-Term Performance Evaluation

Deep geological disposal is widely recognized as the most reliable strategy for the long-term isolation of high-level radioactive waste (HLW). In granitic host rocks, fractures in the near-field represent the primary pathways for groundwater flow and potential radionuclide migration. The self-sealing capacity of carbonate-filled fractures, along with its long-term effectiveness, plays a critical role in maintaining the integrity of the multi-barrier system and ensuring repository safety. Near-field fractures undergo complex thermo–hydro–mechanical–chemical (THMC) coupled evolution driven by excavation-induced disturbances, decay heat, groundwater saturation, and ongoing water–rock interactions. Within the confined fracture spaces, carbonate minerals may persistently undergo precipitation–dissolution cycling and micro- to nanoscale structural reorganization, resulting in progressive reductions in fracture connectivity and hydraulic transmissivity. However, existing studies have largely focused on short-term sealing effects, with limited systematic understanding of the long-term safety functions. In this context, this study comprehensively investigates carbonate-induced self-sealing in granitic fractures within the near-field of a repository under realistic THMC-coupled conditions. We elucidate the micro- and nanoscale heterogeneous precipitation characteristics governed by non-classical nucleation pathways, reveal how dynamic precipitation–dissolution equilibria facilitate ongoing reductions in fracture transmissivity, and propose a multi-dimensional framework for long-term hydraulic, mechanical, and chemical performance assessment. Our findings demonstrate that carbonate self-sealing operates as a dynamic, reorganizing, and multi-mineral cooperative mechanism rather than a static, one-directional process. Its core safety function lies in the sustained suppression of fracture transmissivity. The mechanistic insights and evaluation framework proposed in this study provide a foundation for integrating natural carbonate self-sealing with engineered barrier system design, thereby improving fracture control, advancing long-term safety assessment, and optimizing the design of HLW deep geological repositories.