Advances in formation of bentonite colloid and its stability in near-field of high-level radioactive waste repository

The compacted bentonite used as a buffer/backfill material for deep geological repositories will release colloid into groundwater under erosion, while stable and mobile colloids may promote the migration of radionuclides, thereby threatening the long-term safety of the repositories. By elaborating the definition and characterization of colloid of bentonite, this study comprehensively reviews and summarizes the research results of generation tests, generation mechanism and stability of colloid. The results show that the bentonite colloid is generated by the continuous hydration of the montmorillonite crystal layer and the separation of free colloidal particles into the aqueous solution. The colloidal concentrations and sizes are significantly affected by clay characteristics, groundwater dynamics and chemical conditions, and the colloidal stability is obviously affected by clay characteristics, groundwater chemical conditions and temperature. The colloid generation test devices include static and dynamic forms which cannot achieve real-time automatic monitoring at present. The classic DLVO theory does not consider the effect of pH on the edge charge of the colloid and the effect of multivalent counter ions, and there are some limitations in the application of montmorillonite clay colloid generation model and colloid stability prediction. Finally, some suggestions are proposed for further researches.