Experimental Research On Ion Exchange-Reverse Osmosis Treatment Of Low-level Radioactive Wastewater

With the completion and commissioning of the world's first third-generation nuclear reactor AP1000,the disposal of radioactive waste has attracted more and more attention.Nuclear facilities generate large amounts of radioactive waste water during operation,maintenance and decommissioning.Such wastewater must be removed from radionuclide and separated from boric acid before being released into the environment.Based on the in-depth study of low-level radioactive wastewater treatment methods,this paper proposes to separate the radionuclide and boric acid from water by ion exchange-reverse osmosis technology,and further separate the boric acid from water to realize the harmless treatment of nuclear industrial waste.The main research contents of this thesis include experimental research and mechanism analysis of two stages:radionuclide removal by ion exchange and boric acid removal by reverse osmosis.The exchange capacity was used as an indicator to compare the three ion exchange resins of 732,D72 and D001,and it was finally proved that 732 resin exchange capacity is optimal.The significant influence factors of resin exchange capacity were investigated by tests of solid-liquid ratio,reaction time,temperature and initial concentration.The analysis proves that the ion exchange rate is mainly affected by the liquid film diffusion process,and the exchange reaction is a monolayer reaction.When Co and Mn ions compete for the chemical sites on the resin,the competitiveness of Mn ions is slightly stronger,and boric acid has no effect on the competitive reaction.The effect of ion exchange on the removal of radionuclides was measured by the overall removal rate and exchange capacity.The result shows that the dynamic exchange capacity of Co and Mn ions is basically consistent with the static exchange capacity under single system conditions.The breakthrough time,exchange capacity,and overall removal rate increase as the height of the resin column increases or the influent flow rate and the influent concentration decrease.The main reason is the increase of the hydraulic retention time of the target and the decrease of the resin's pollutant load and the mass transfer driving force.The exchange rate constant and exchange capacity of the resin under different operating conditions were calculated by using the BDST model,which was used to effectively predict the operation of the fixed bed when only the influent flow rate or the influent concentration was changed.X-ray energy spectrum and Fourier transform infrared spectroscopy were used to analyze the proportion of each element before and after the reaction and the change of the wavelength of each chemical bond absorption peak.The effect of reverse osmosis on the removal of boron was measured by the removal rate of boron acid.The result shows that the most significant factor influencing the reverse osmosis denitrification rate is the influent pH.The removal rate of reverse osmosis increases with the increase of the influent pH,the increase of the operating pressure and the decrease of the recovery rate.The polyhydroxy sorbitol can effectively enhance the removal effect and achieve a higher removal rate of boric acid at relatively low pH conditions.The irreversible thermodynamic model was used to calculate the reflection coefficient and permeability coefficient of H₃BO₃ and H₂BO₃^-components,which reasonably explained the mechanism of reverse osmosis removal rate of boric acid with influent pH.Also,the removal rate of boric acid in reverse osmosis membrane at 20?under different transmembrane pressure difference and influent pH was predicted.Through this test,the experimental data of the feasibility operation was obtained,and the relevant mathematical model was used to predict and evaluate the removal in the actual production process,laying a technical foundation for the combined process for the treatment of boron-containing low-level radioactive wastewater.