XAFS And HERFD-XAS Study On Uranium-Contained Mixed Oxide

As a clean and renewable energy source,nuclear energy has received increasing attention with the growing of the energy demand.Nuclear power can effectively alleviate the greenhouse effect,reduce environmental pollution and provide a sustainable energy supply.Uranium-plutonium and uranium-thorium mixed oxides are the two common nuclear fuels in nuclear power plant reactors.They will become complex spent fuel containing decades fissile elements and several actinides elements.Some of the lanthanide and actinide products would form the solid solution with the fuel matrix,which significantly effects the fuel performance.Therefore,the study on the structure of nuclear fuel and spent fuel is an important basis for nuclear fuel,fission product separation,radioactive waste treatment and disposal.Due to the radioactivity and complexity of mixed oxide fuel,such researches require advanced detection methods with permeability and elemental selectivity.With the continuous development of synchrotron radiation technology,X-ray absorption spectrum?XAS?has become an important spectroscopy method for studying the structure of materials.This method can obtain the local structure information such as the valence state of the target element,the coordination numbers and the bond lengths.High energy resolution fluorescence detected X-ray absorption and emission spectrum?HERFD-XAS and XES?are an advanced spectroscopy based on X-ray absorption spectrum.This method is to record a specific fluorescence signal by the analyzing crystals,which can obtain a more accurate absorption or emission spectrum.High energy X-ray has strong penetrability,and the absorption edge of every element is unique.Therefore,X-ray absorption fine structure?XAFS?,HERFD-XAS and XES have broad prospects in the study of nuclear material.The main research work of this thesis is supplied by Shanghai Synchrotron Radiation Facility and Shanghai Institute of Applied Physics.The uranium-cerium and uranium-thorium mixed oxide are prepared to characterized by XAFS and HERFD-XANES.Combined with XRD technology,the structural changes of uranium,cerium and thorium in fuel cycle were systematically studied.The main research work is as follows:?1?A high-temperature reduction equipment was built to reduce the uranium-containing sample at 1300? in hydrogen atmosphere.A series of U-Ce mixed oxides were prepared by the equipment for structural characterization.The experimental results show that the series of U-Ce mixed oxides form a solid solution with the lattice parameter between UO₂ and CeO₂.In,U and Ce atoms have different interactions with the local structure in U_1-xCe_xO₂ solid solution.When the sample has low Ce content?<35%?,the addition of Ce has little effect on the U-O₈ cube,and leads to an increase in structural disorder.If the sample has high U content?>35%?,the presence of Ce tends to reduce the oxygen deficiency around U,which reduces the U-O₈ sublattice number and the overall disorder.In contrast,when U is added to CeO₂,the disorder of Ce-O₈increases as the U content increases.?2?In order to simulate the unstable oxidation of spent fuel in harsh environments,a series of Ce-doped U₃O₈ sample was prepared by co-precipitation method and their structure were analyzed.The experimental results show that a small amount of Ce?<20%?could dope into the U₃O₈ matrix,and replace the U-site.This reaction will change the U₃O₈ matrix from the C2mm space group to the P6?2m space group.Compared with the U₃O₈ sample without Ce,the unit cell volume of the(U_1-yCe_y)₃O_8±?samples slightly increased,and the average valence state of U ions increased slightly for charge balance.?3?In order to study the structure of nuclear fuel,Th_1-xU_xO₂ mixed oxides were prepared by coprecipitation method.The phase information of mixed oxides and the local structure of Th and U atoms were obtained by XRD and XAFS methods,respectively.The samples with different uranium/thorium ratios are fluorite solid solution.When the uranium replaces the thorium site,it reduce the volume of the adjacent Th-O8 cube.Further HERFD-XAS experiments revealed that the white line of Th L₃ edge split into two peaks.The result,together with the local-density approximation,shows that the crystal field around the Th⁴⁺ion and the Th-O covalent bond in Th_1-xU_xO₂ are sensitive to the local symmetry of thorium atoms.The addition of uranium causes distortion of the Th-O₈ cube,reduces the crystal-field splitting between the thorium 6d e_g and t_2g orbitals and the overlap between the thorium 6d t_2gg and oxygen 2p orbitals.The above results based on the synchrotron radiation XAFS and HERFD-XAS method to study the structure of U-Ce and U-Th mixed oxides,illustrate the feasibility and reliability of XAFS and other methods for the coordination chemistry of the actinides.This paper has certain guiding significance for the separation and post-treatment of spent fuel and its products.