Self-sealing behavior of bentonite-based materials in high-level radioactive waste disposal: A systematic review

During the construction of high-level radioactive waste disposal repository, bentonite-based materials (BM), as the preferred type of buffer materials, are usually pre-compacted into bricks and then emplaced between hostrock and metal canister. This construction procedure inevitably forms a certain volume of technological voids between BM bricks and host-rock, BM bricks and BM bricks, as well as BM bricks and metal canister. These technological voids would be spontaneously sealed by the hydrated and swollen bentonite when groundwater infiltrates from host-rock, and such phenomenon is referred to as "self-sealing". This review synthesizes recent research findings devoted to the self-sealing behavior of BM. First, representative experimental approaches that have been adopted for investigating self-sealing behavior both in laboratory scale and in-situ scale are introduced. Subsequently, key research efforts associated with self-sealing dynamics, hydro-mechanical responses and theoretical models for self-sealing behavior are summarized and remarked respectively. Here, self-sealing dynamics are tentatively divided into three modes based on reported experimental observations, including fillinghomogenizing, cracking-filling-homogenizing and "Filling-fracturing" cycles-homogenizing. Emphases for the hydro-mechanical responses to self-sealing behavior are placed on saturation kinetics and swelling pressure. Technological voids can accelerate the saturation degree evolution and lead to a higher saturated hydraulic conductivity than the intact sample with an equivalent global dry density, which is suspected to be caused by the zone close to initial technological voids acting as preferential water flow channels. Technological voids impact the swelling pressure evolution by altering boundary conditions, but whether they will affect final pressure is related to the type of technological voids, the volume ratio of technological voids, the global dry density and the orientation of swelling pressure, etc. Theoretical models for self-sealing behavior involve the constitutive models of BM and the modelling of technological voids, whereas existing models cannot neither reproduce the cracking and fracturing phenomena nor precisely reflect the hydraulic and mechanical properties of technological voids. Finally, key research recommendations for future work are proposed. This review aims to shed the light for investigating self-sealing behavior of BM to secure the long-term safety of engineered barrier in high-level radioactive waste disposal.