Synthesis,Structure And Properties Of Thorium And Uranium Compounds

Thorium-based Molten Salt Reactor(TMSR)belongs to the fourth-generation new nuclear energy system.It mainly uses fluoride eutectic salt as the carrier for transportation and cooling.The nuclear fuel is uniformly dissolved and dispersed in the molten salt circuit,which is safe,economical and efficient.However,the hightemperature operation of the molten salt circuit makes the fuel salt undergo complex physicochemical changes,which make the molten salt system have very complex structural characteristics.Therefore,to accurately obtain the microstructural characteristics of the fuel salt during operation,which is very important for understand the physicochemical behavior of Th and U in different molten salt environments.For example,by in situ acquisition of Th and U coordination structure data to study the properties of neutron moderation and absorption,which could ensure the safe and stable operation of the fuel salt in the high temperature and strong corrosive molten salt environment.Secondly,there is an important follow-up problem in the operation of Thbased molten salt reactors:how to stably handle highly radioactive nuclear waste,including long-lived actinides such as 239Pu,232Th,and 233U,and short-lived fission products such as 137Cs and 90Sr.The research of matrix materials for long-term storage of highly radioactive nuclear waste is an important factor for the sustainable development of nuclear energy.Therefore,researching the structure of actinide nuclides such as Th and U in storage materials helps to understand their physical and chemical properties,which is of great significance for the design of material systems.Aiming at the above scientific problems,this paper studies the Th and U structure in molten salt systems and storage materials through the design of characterization methods and material synthesis methods.The main contents are as follows:This work is the first to design a high temperature in situ X-ray absorption fine structure(XAFS)spectroscopy experiment,to research the local structure of Th in the ThF4-LiF and ThF4-LiF-BeF2 molten salt systems.Combined with molecular dynamics(MD)simulations to evaluate the effect of microstructure on the transport properties of molten salts.It means that the presence of Be facilitates the exchange of Th,F ions.Experimental and theoretical XAFS analysis results indicate the existence of structures of intermediate-range ordered coordination shells(Th-F2nd and Th-Th)in molten salts,which play an important role in the transport properties.MD simulations were used to evaluate the bonding modes in molten ThF4-LiF and ThF4-LiF-BeF2 mixtures,which confirmed the experimental conclusions.Then,the work conducts related research on the structure of uranium in nuclear waste storage materials Research on the structure of uranium in nuclear waste storage materials.First,an experimental method was designed in this work to successfully synthesize Nd2Zr2O7 pyrochlore materials using an optimized molten salt method.Then,the Nd or Zr site substitution of uranium is realized by optimizing the experimental conditions,which is used to study the structural properties of uranium in the material matrix.XRD results show that uranium ions can replace the lattice sites of Nd or Zr,respectively,to form two systems of pure Nd2-xUxZr2O7+δ(0<x<0.2)and Nd2Zr2yUyO7+δ(0<y<0.4).The results of Raman spectroscopy show that with the increase of uranium concentration in the system,the host material undergoes a phase transition process from pyrochlore phase(Fd-3m)to defective fluorite phase(Fm-3m).XAFS analysis showed that uranium was present in the high valence state U6+in all samples.In order to maintain the charge balance when U6+replaces Nd3+or Zr4+,additional oxygen was introduced into the system,accompanied by the formation of local structural distortion of uranium-oxygen and zirconium-oxygen.However,the regular cubic fluorite structure is maintained even with 20 mol%uranium substitution,indicating that either the Nd or Zr cation sites of the Nd2Zr2O7 matrix have good solubility for uranium.This work is the first time to use the optimized molten salt method to synthesize a storage material system of uranium-substituted Nd2Zr2O7 at Nd or Zr sites.The long-range and short-range structures were systematically studied,and the inclusion of high-valence uranium at A or B sites in the A2B2O7 pyrochlore structure was confirmed.On this basis,a higher concentration doping experiment of uranium in the lanthanide zirconate material system was carried out,the state of uranium was deeply analyzed,and the corresponding mechanism research was carried out on the slight change of the host material structure.First,the experimental method was further optimized,and the uranium-substituted La2Zr2O7 pyrochlore host materials were successfully synthesized:La2-xUxZr2O7+δ and La2Zr2-yUyO7+δ.By comparison,the traditional solid-phase method was used to achieve uranium substitution at the La and Zr sites,respectively,and the solubility was(0<x<0.2)and(0<x<0.2),The optimized molten salt method achieves high-concentration uranium substitution at La sites(0<x<0.2)and Zr sites(0<y<2.0),respectively.Among them,there is a new structure La2U2O7+δ.The results of XRD,Synchrotron-XRD and Raman spectroscopy show that with the increase of uranium concentration in the La2Zr2O7 pyrochlore material,the long-range ordered pyrochlore phase remains stable,while the local oxygen environment is distorted to varying degrees due to the substitution of uranium.The local structure characterization analysis by XAFS shows that when uranium is substituted at La or Zr sites,it exists as UO66-at different concentrations.With the change of oxygen environment,the U-O structure has the characteristics of disorder with the increase of uranium concentration.In addition,the fitting of U-O bond lengths by EXAFS shows that the substitution of the local oxygen environment at the La or Zr sites is slightly different,confirming that in the pyrochlore system,the optimized molten salt method can make the uranium substitution of specific site.This conclusion is confirmed by U M4-edge HERFD-XANES spectroscopy.For the new structure La2U2O7+δ,we use HAADF-STEM to probe the general nature of La,U site exchange,and this disorder confirms the formation of a fluorite phase(Fm-3m)isostructural with pyrochlore.Finally,DFT calculation was carried out on the relative stability of La and Zr sites in La2Zr2O7 material after being substituted by uranium,and it was verified that doping with high concentration of uranium makes the structure more fluorite phase.