| The fission gases known as xenon and krypton isotopes are mainly released from nuclear power plants and tests,which are the products under 235U,233U and 239Pu fission reactions with thermal or fast neutrons,respectively.The yield of artificial radioactive xenon isotopes is about 14 times than that of krypton isotopes.The xenon and krypton isotopes such as 126Xe,128Xe,78Kr and 80Kr are naturally existed,which is trace amount.These total xenon and krypton isotopes were mixed in the air,whose concentration is0.087 ppm and 1.14 ppm,respectively.The current 85Kr accumulated in the atmosphere was artificially released.Xenon and krypton belong to inert gases which are rare,colorless,and odorless.Because of the saturated electrons in outer shell,xenon and krypton show stable chemical and physical properties and are usually separated and collected by physical methods including low temperature distillation,solvent absorption,fixed-bed adsorption,high-pressure storage and membrane separation.Fixed-bed adsorption with activated carbon and decay in pressurized-tank are commonly used in most nuclear power plants for radioactive xenon and krypton isotopes treatment.Though the two treatment methods are economic and easy to operated,it is difficult to separate xenon from krypton effectively and the long half-life radioactive 85Kr discharged from the reactors directly.Therefore,the separation and purification of xenon and krypton isotopes from gas treatment system is a key technology for nuclear power plants.The technology can change waste gases to treasure and establish sustainable development.Based on the operation condition of thorium-based molten salt reactor and fixed-bed adsorption method,the study focus on the functional material with high xenon and krypton selectivity.In order to find the relationship between the pore size and the adsorption capacity of krypton and xenon,the static and dynamic adsorption experiments are conducted with activated carbon in nuclear grade.The results shows that the pore size in 4-10（?）range gives positive effect in xenon and krypton separation.According to the pore size effect on adsorption and separation of krypton and xenon,the appropriate flexible or rigid organic carboxylic acid ligands were selected to synthesize metal-organic frameworks（MOFs）rationally on the base of theory and experiments.MOFs with high specific surface area,topological diversity,pore size adjustability and catalytic sites,which are widely developed in gas purification,separation,catalysis and storage of high-tech industrial fields.In this study,Ce-SINAP-1,Ca-SINAP-1 and FMOF-Co were prepared by solvent-thermal andγ-ray irradiation synthesis method.Samples were characterized through the specific surface area,pore volume,thermogravimetric and powder X-ray diffraction analysis.The experiment results showed the three MOFs all belong to microporous materials,whose pore size are in the suitable range for Xe,Kr adsorption.The adsorption and separation properties of the three materials were experimentally studied by static and dynamic adsorption tests with argon,krypton and xenon.The connection of organic ligand and metal ion forms diamond windows in Ce-SINAP-1,which supply the pore effect to separate krypton and xenon.After high temperature and vacuumized activation,binding water around Ca2+was removed and metal open-sites appeared.Ca-SINAP-1 showed excellent separation performance of xenon,krypton and argon at appropriate pore size（4-10（?））.The results of dynamic adsorption experiments show that this material can effectively separate xenon from krypton and argon.Theoretical simulation showed that Ce-SINAP-1 separated xenon and krypton through pore size effect on the structure.Ca-SINAP-1 separated xenon and krypton through polarization effect contributed by the coordination oxygen atoms with its lone pair electrons.Besides,the metal open-site of calcium and the frame structure contributed to the xenon adsorption.In addition,cerium-based and calcium-based MOFs have good thermal stability and can withstand certain dose ofγ-ray irradiation.When the silver loading amount was 0.93%on Ca-SINAP-1,the Xe uptake the selectivity was enhanced.The silver nanoparticles attached on the surface enhanced the interaction force between the samples and xenon,thus effectively enhanced the adsorption and separation of xenon.FMOF-Co showed a reversal phenomenon of xenon adsorption behavior with decreasing temperature.At room temperature and pressure,the adsorption capacity of xenon is twice than that of krypton.When the temperature drops to 233 K,the adsorption capacity of krypton is 6.3 times than that of xenon,and 8.2 times than that of argon,respectively.This reversal phenomenon indicates that the separation of krypton and xenon can be achieved by different temperature.The experimental study and theoretical calculation in this dissertation show that the pore size,the frame topology,the metal open-site and the noble metal loading have positive effects to enhance the separation of krypton and xenon,These three materials can be used in the radioactive off-gas treatment system of nuclear reactors for xenon and krypton recycling,storage instead of direct discharge.Therefore,functional materials with high selectivity of xenon-krypton is significant for radioactive exhaust gas treatment system of nuclear reactors. |
