Investigation on the Structural Incorporation of Various Dopants into Uranium Oxides in Consideration of Application Requirements as Reference Materials

In the last few years, an aerosol-based process has been implemented and established in the laboratories of Forschungszentrum Jülich to produce microparticulate uranium oxide reference materials. These microparticulate reference materials with well-defined properties are needed to consolidate the quality controls of the analytical methods used in particle analysis in nuclear safeguards. As a result, these reference materials must fulfill certain requirements. These requirements are specified by the International Atomic Energy Agency (IAEA) to be similar to the U-containing microparticles collected by an IAEA safeguards inspector during in-field verification activities. These so-called environmental samples are analyzed for their isotopic composition by the IAEA’s Office of Safeguards Analytical Services and their dedicated Network of Analytical Laboratories (NWAL). In order to further develop analytical methods and quality control of the analytical results from particle analysis to even detect small quantities of fission products, the microparticulate reference materials must be refined. For this purpose, first attempts were made to synthesize Nd-mixed uranium oxide microparticles. However, due to yield limitations in the µg range, no state-of-the-art analytical methods can be used for the characterization of these materials and the corresponding chemical stability of the mixed microparticles. Therefore, a co-precipitation method was adapted to synthesize bulk-scale comparison materials with various lanthanides and thorium in order to unravel the incorporation mechanism of those dopants into the uranium oxide structure in depth. Through TG-DSC studies on the mixed bulk-scale materials, the temperature range of phase transitions from UO₃ to U₃O₈ were identified and analyzed in dependence on the used dopant. By additional systematic structural investigations of the long-and short-range order phenomena with XRD and Raman, IR, and XAS, respectively the obtained doped UO₃ and U₃O₈ materials could be characterized in more detail. This presentation will show results regarding the incorporation of lanthanides and thorium into the uranium oxide structures and the corresponding phase transformation of the orthorhombic U₃O₈ to the hexagonal U₃O₈ crystal structure. These results will be integrated into the particle production process to design well-defined microparticulate mixed oxide reference materials.