Abstract
The safety assessment for the long-term operation of a deep repository is an essential work for the engineering of geological disposal for high-level radioactive waste (HLW). Bentonite selected as a barrier/backfill material in repositories, can be exposed to the chemical effects of alkaline site water owing to cement degradation, which poses a great threat to the long-term safety of the repository.
This work is focusing on the swelling property of bentonite with alkaline pore water in order to investigate the evolution of vertical strain of compacted Gaomiaozi bentonite under the simulated long-term environments of Chinese HLW repository. Fourier transform infrared spectroscopy, X-ray powder diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy tests were utilized to analyze the bentonite after it reacted with the pore water solutions.
Results reveal that the swelling strain is significantly influenced by the composition and concentration of the solutions, and that swelling time decreases as temperature increases. In the secondary swelling stage of the bentonite hydrated by solutions containing K+ , the swelling strain decays because K ions enter the montmorillonite crystal layer and generate low-swelling or non-swelling minerals.
The solutions (particularly high alkaline solutions) containing K ions strongly decrease the swelling performance of bentonite, suggesting that alkaline cement water can degrade the swelling property of the buffer material in a high-level radioactive waste (HLW) disposal system.