Study On Direct Solidification And Stability Of Radioactive Zeolite Waste

With the development of nuclear energy,a certain amount of radioactive waste is inevitably produced.As one of the important adsorbents,zeolite is selected for the removal of radioactive waste in the field of nuclear energy.Nevertheless,when zeolite is used to treat radioactive waste with complex components,radioactive zeolite will be produced.Due to the existence of radioactivity and metal toxicity peaks,the used zeolite cannot be reused,and finally radioactive zeolite waste is formed.If these radioactive wastes are not properly treated,they may leak out and pollute the environment,bringing harm to human existence.In this paper,natural zeolite was used as the substrate,Nd₂O₃and Ce₂O are used as substitutes for radioactive An³⁺and An⁴⁺respectively,and UO₂（NO₃）₂·6H₂O is used as the raw material,and a series of simulated radioactively polluted zeolite solidified bodies were successfully prepared.The phase,microstructure and morphology of radioactive contaminated zeolite were tested by XRD,FT-IR and SEM-EDS,and the physical and chemical properties of the cured body were tested and analyzed.At the same time,the theoretical simulation calculation of the solidified body was carried out in combination with molecular dynamics,and the microscopic evolution law of the solidified body was obtained by calculating the radial distribution function,mean square displacement,bond length and angle of the solidified body.The main results obtained by combining experimental research and theoretical simulation are as follows:（1）The sintering process of zeolite at different temperatures（1200°C,1300°C,1400°C and 1500°C）was explored.By calculating the crystallinity of the solidified body,the optimal sintering process of the radioactively contaminated zeolite is determined.The results show that when the sintering process is held at 1500°C for 1 h,the solidified body shows a complete amorphous phase.Finally,the sintering process for simulating radioactively contaminated zeolite was determined to be sintering at 1500°C for 1 h.（2）The simulated An⁴⁺radioactively polluted zeolite can be solidified in the form of glass and ceramic double containment,and its maximum solid solubility can reach 43wt.%.The SEM-EDS test results of the maximum solid solubility solidified body show that the solidified body has obvious crystalline regions,and the EDS results show that O,Si and Ce are enriched,which is consistent with the crystal elements in XRD.Vickers hardness and density can reach 7.44 GPa and 3.34 g·cm^-3respectively.The leaching results on the 28th day showed that under the different leaching environment of p H=4,7 and 10,the leaching rates of Ce elements in the solid with doping amount of 43 wt.%were 1.56×10^-3g·m^-2·d^-1,2.40×10^-5g·m^-2·d^-1and 3.63×10^-4g·m^-2·d^-1,respectively.The solidified body of radioactive contaminated zeolite was simulated by the method of molecular dynamics.The MSD results show that O,Si and Al form stable[Si O₄]and[Al O₄]tetrahedra in the solidified body,and Ce is stably fixed in the solidified body.The results of RDF show that the interaction among Si,Al and O is stronger in the glass network structure.The bond angle distribution results show that the O-Al-O bond angle distribution is wider and smaller than the O-Si-O bond angle distribution,indicating that the strength of the[Al O₄]tetrahedron is weaker than that of[Si O₄].（3）The results of solidification simulation of An³⁺and An⁴⁺co-doped contaminated zeolite at different temperatures show that the solid solution of Nd₂O₃is 15 wt.%and the solid solution of Ce O₂is 10 wt.%of the sintered solidified body at 1400°C.When the sintering temperature rises to 1500°C,the solid solubility of the solidified body to Nd₂O₃can reach 20 wt.%,and the solid solubility to Ce O₂can reach 35 wt.%.FT-IR test results show that the structure of the glass solidified body is mainly composed of Q³and Q⁴,and its content increases with the increase of sintering temperature.The results of the leaching experiment after 28 days showed that the chemical durability of the sintered sample at1500°C was better,and the normalized leaching rates of Ce and Nd elements in the solidified body are both in the order of～10^-7g·m^-2·d^-1.（4）The solid solution amount of U in the simulated U-polluted zeolite solidified body can reach 50000μg/g.The results of SEM-EDS showed that the cured body exhibited a glass-ceramic structure.With the increase of the content of U,the Q³+Q⁴of the solidified body also increases,indicating that the increase of U can increase the content of bridge oxygen bond and improve the stability of the solidified body.At 42 days,the leaching rate of U in the solidified body is 7.45×10^-6g·m^-2·d^-1with the doping amount is 50000μg/g.