Multiphase flow and heat transfer-driven fracture in saturated buffers for a nuclear waste repository: A novel phase-field cohesive zone model

During the operational lifespan of high-level nuclear waste repositories, significant gas generation is anticipated through various physicochemical processes, including metal corrosion, water radiolysis, and microbial degradation. The subsequent migration of these gases through a saturated buffer poses significant challenges to the integrity of the engineering barrier. ‌Given the complex coupled interactions among gas, liquid, and solid phases within buffer materials during gas breakthrough processes under high-temperature environments, this study developed a novel phase-field cohesive zone model that captures fracture propagation jointly driven by multiphase flow, temperature, and pore pressure. The relative permeability contrast between the matrix and the fracture region is considered to capture the sustained gas breakthrough paths. The model demonstrates enhanced robustness through stabilized fluid source terms, particularly when employing low-order quadrilateral elements. Model validations are performed against both analytical solutions and numerical benchmarks. Field-scale simulations revealed that thermal effects on gas properties significantly accelerate gas breakthrough phenomena. The contact stiffness between the buffer material and the natural barrier has a significant impact on gas breakthrough, which is closely related to the rate of gas generation. Material heterogeneity governs preferential breakthrough pathways. This work establishes fundamental theoretical frameworks for optimizing buffer material composition and informing the design of deep geological repositories. The findings provide critical insights into gas migration mechanisms in nuclear waste containment systems.