Theory Of Swelling Properties And Mechanism On Swell Attenuation Of Bentnoite In Sal Solution

The deep geological disposal concept has been accepted by many countries to isolate the high level radioactive waste from the biological environment. Bentonite is used as the main material of buffer and backfill materials in the repository because of its low hydraulic conductivity, high swelling property, good adsorption capacity of nuclide, and etc. During the operation of deeply repository, groundwater will intrude through surrounding cracks rock and infiltrate into buffer and backfill materials, and then groundwater will influence the swelling properties of bentonite and the the safe operation of the repository system. Therefore, predictions of the swelling behavior and the swelling attenuation of bentonite in groundwater are the key technical issues in the construction and operation of deep nuclear wastes disposal system. In this paper, according to laboratory tests and measurements, combining with theoretical analysis and numerical simulation, the swelling properties of bentonite in solution is studied, a new calculation method for swelling in aqueous solution is proposed, the mechanism on swelling attenuation influenced by aqueous solution is analyzed, and the swelling attenuation is calculated by the proposed model. Then, the variation of swelling properties of bentonite in nuclear wastes repository is simulated numerically. These studis should provide a scientific basis for the building of deep nuclear wastes disposal system. The main work is as follows:(1) N2 molecule adsorption tests of bentonite samples are producted to measure the surface fractal dimension of bentonite. According to the N2 molecule adsorption, the surface fractal dimension of compacted bentonite swelling in distilled water and Na Cl solution with different concentration is measured, it is found the surface fractal dimension of compacted bentonite is basically the same as that of bentonite powder, it is suggesting salt solution has few influence on the surface fractal dimension of bentonite.(2) Common consolidometers is adorpted for swelling tests. A series of swelling deformation and swelling pressure tests of commercial bentonite were producted in distilled water and Na Cl solution with different concentration. The swelling characteristics of bentonite is closely related to the applied vertical pressure, the initial dry density of samples and the concentration of salt solution. The osmotic suction of Na Cl solution has strong inhibition on the swelling characteristics, as the concentration is higher, the swelling property is lower.(3) The fractal model for bentonite swelling is established. Based on the effect of surface absorption in swelling process and the fractal nature of bentonite surface, the expression of water volume absorped in swelling process is proposed, and then the theoretical formula for swelling deformation and swelling pressure is derivated. The montmorillonite swelling coefficient K in fractal model is calculated by combination with DDL model in suction swelling phase in distilled water. In salt solution, based on the fractal theory of bentonite surface, the effective stress induced by osmotic suction of pore water is deduced, and the swelling model in salt solution is proposed, the model is verified by swelling tests in salt solution.(4) The mechanism on swelling attenuation of GZM bentonite in chloride salt solution(Na Cl, Ca Cl2 and Mg Cl2) and Na OH solution is revealed. After mixture of bentonite with chloride solution, the ion exchange is the main chemical reaction process, Ca2+ and Mg2+ in solution will change the Na?montmorillonite into Ca? or Mg? smectite with lower swelling capacity, leading to swelling attenuation. When bentonite amended with Na OH solution, the the high alkalinity solution has strong dissolution effect on montmorillonite. The montmorillonite is dissoluted, the non-swelling mineral precipitated, and the swelling attenuation occurs as a resulte. Based on the mechanism of swelling attenuation, the regulation of swelling attenuation induced by osmotic suction, ion exchange and montmorillonite content is predicted by the swelling fractal model.(5) Basd on the THMC four coupling interaction and mutual influence on the backfill/buffer materials in nuclear waste repository, the THMC multi–field coupling equationes is established. The process of water absorption and swelling, and the long time scale of swelling attenution under multi–field coupling is simulated by the softwares of COMSOL Multiphysics and Phreeqc. The evolution regularity of water adsorption and swelling over time is analyzed, the osmitc sucion, montmorillonite content, swelling coefficient K, effective stress and swelling pressure attenuation of saturated bentonite with time passing is calculated, and the quantitative evaluation method for swelling attenuation is proposed.