Synthesis Of High Purity UF₃ And Its Analysis Method

Uranium trifluoride(UF₃)is a reducing and stable uranium fluoride in molten salts,which can inhibit the corrosion of structural metals in high-temperature fluorides of molten salt reac-tors,prevent intergranular cracking of alloys caused by the fission product tellurium,and par-ticipate in the reconstitution of reactor fuel salts of molten salt reactors.The role of UF₃ in extending the lifetime of structural materials has been confirmed in the MSRE experimental reactor at Oak Ridge National Laboratory.However,the UF₃ content requirements for different molten salts and structural materials are different and need to be further investigated,while the UF₃-based fuel reconstitution technology is still in the laboratory research stage,and the tech-nical details are yet to be refined.Since UF₃ is not commercially available,this paper prepared high purity UF₃ under relatively mild laboratory conditions and investigates the synthesis mech-anism of the reaction;meanwhile,this paper establishes a method to determine the purity of the obtained UF₃ powder based on selective dissolution,which is simple to operate and highly accurate.The main works of this thesis are as follows:1.uranium metal（U）is converted to uranium hydride（UH₃）by hydrogenation,and based on the high reactivity of UH₃,high purity UF₃ was synthesized by solid phase reaction of UH₃and uranium tetrafluoride（UF₄）in argon atmosphere,thus reducing the temperature and time required to prepare UF₃.The samples at different temperature points and different holding times during the synthesis of UF₃ were analyzed by XRD,XPS,DSC and SEM,and the effects of reaction temperature and reaction time on the phase changes of the synthesized samples were investigated.Combining DSC data and XRD phase analysis,the reaction mechanism of UH₃with UF₄ was determined,and we found that both UH₃ and its decomposition product,i.e.,U,react with UF₄,and the starting temperature of each reaction was obtained.The kinetic model of the reaction of UH₃ and UF₄ was investigated by fitting the results of the yield of UF₃at different holding times in the context of the reaction phenomena and the microscopic morphol-ogy of the products,and it was found that the reaction was consistent with the 3-D interface reaction（spherical particle）model.The simple preparation of the pure phase UF₃ could provide material of high quality for further studies of the physicochemical properties and the behavior of UF₃ in the related molten salts.2.A simple and accurate analytical method for the determination of UF₃ purity using plasma atomic emission spectroscopy（ICP-OES）was established based on the selective disso-lution of the mixture of UF₃ and UF₄ by cerium sulfate（Ce（SO4）2）solution,which avoids the difficulties of the previously reported method that requires an oxygen-free environment,com-plicated and longtime operation.The interference of cerium（Ce）element in the matrix to the emission spectrum of U element was eliminated by the multispectral fitting technique（MSF）,and the linear correlation coefficient of the working curve was close to 1,and high precision and recovery were achieved.For different mixed samples,the relative error is less than 2%,which met the experimental requirements and was suitable for analyzing the purity of UF₃ in the mixture of UF₃and UF₄,and it also laid the foundation for the subsequent analysis of U³⁺and U⁴⁺concentrations in fluoride molten salts.