The International Atomic Energy Agency implements safeguard measures to verify the compliance of Member States to their international legal obligations using nuclear material and technology only for peaceful purposes. These safeguard measures, i.a., include analytical measurements of individual micrometer- and submicrometer particles taken by the IAEA on swipe samples during safeguard inspections at nuclear facilities. To ensure the quality control of the analytical results from particle analysis and to further develop mass spectrometric analysis methods, microparticles with well-defined properties as microparticulate reference materials are required. Therefore, mixed lanthanide/uranium oxide microparticles were produced as a first step towards composite reference materials with small amounts of fission products, Pu or Th. A deep understanding of the incorporation mechanisms of dopants into U3O8 structure is essential in this regard. Therefore, bulk-scale comparison materials were produced and doped with lanthanides by co-precipitation methods and systematically investigated by TG, XRD, and Raman. These results will be integrated into the particle production process to design well-defined microparticulate mixed-oxide reference materials.

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Structural incorporation of lanthanides (La, Eu, and Lu) into U3O8 as a function of the ionic radius

The International Atomic Energy Agency implements safeguard measures to verify the compliance of Member States to their international legal obligations using nuclear material and technology only for peaceful purposes. These safeguard measures, i.a. , include analytical measurements of individual micrometer- and submicrometer particles taken by the IAEA on swipe samples during safeguard inspections at nuclear facilities. To ensure the quality control of the analytical results from particle analysis and to further develop mass spectrometric analysis methods, microparticles with well-defined properties as microparticulate reference materials are required. Therefore, mixed lanthanide/uranium oxide microparticles were produced as a first step towards composite reference materials with small amounts of fission products, Pu or Th. A deep understanding of the incorporation mechanisms of dopants into U_3O_8 structure is essential in this regard. Therefore, bulk-scale comparison materials were produced and doped with lanthanides by co-precipitation methods and systematically investigated by TG, XRD, and Raman. These results will be integrated into the particle production process to design well-defined microparticulate mixed-oxide reference materials. Graphical abstract

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Structural incorporation of lanthanides (La, Eu, and Lu) into U_3O_8 as a function of the ionic radius

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Hydrothermal synthesis of homogenous and size-controlled uranium-thorium oxide micro-particles for nuclear safeguards

The study of uranium oxides at different conditions is of paramount importance in the nuclear field, especially regarding characterization of the spent nuclear fuel behavior in dry storage scenarios. This paper reports results of XRD and Raman analysis on four powdered samples prepared in order to cover a specific stoichiometry range in UO2+x, i.e. x = 0.24, 0.26, 0.28 and 0.30. XRD results reveal a clear increase of the average tetragonal distortion with the increase in oxidation degree, with the main phase detected for all the samples being a weakly tetragonal phase identified as U3O7-z (c/a ≪ 1.032). U4O9 has not been detected in any sample. The Raman study carried out consists of both qualitative and quantitative analysis. The former, where a profile analysis has been performed on the acquired spectra, shows that the most intense bands (centered at ∼455 and ∼635 cm-1) are actually a doublet each, in agreement with a previous experimental study. Moreover, this work shows, for the first time, that the band at ∼160 cm-1 is also a doublet, which makes its classical assignment no longer obvious. The most important and original results from this study are obtained by applying Quantitative Raman Spectroscopy (QRS). This analysis shows that the second contribution at ∼475 cm-1 to the known T2g mode increases its relative intensity with the oxidation degree. This contribution may be related to the tetragonal distortion occurring in the cubic UO2 lattice due to the addition of interstitial oxygen, based on its comparison with the obtained XRD outcomes. In addition, the so-called "defects band" (centered at around 600 cm-1) presents a remarkable kink, of around 20 cm-1, in its Raman shift between UO2.26 and UO2.28. Such behavior might be directly associated with the observed appearance of the stoichiometric U3O7 phase (c/a = 1.032) for UO2.28 and UO2.30.

New insights into the structural transition from UO2+x to U3O7 by quantitative Raman spectroscopy.

An optimized methodology for the determination of the uranium chemical phases in micro-particles by Raman spectrometry within a scanning electron microscope

The analysis of individual micrometre- and submicrometre-sized particles collected by IAEA’s safeguards inspectors on swipe samples during in-field verification activities requires the implementation of a sustainable quality control system such as suitable microparticulate reference materials. To this end, pure and neodymium-doped uranium oxide-based microparticles utilising an aerosol-based particle production process were prepared. SEM/EDX measurements confirmed the monodispersity of the produced microspheres as well as the incorporation of 15 mol% Nd into the compound particles. The timeline of structural investigations mirror the ongoing alteration of particles being stored under laboratory atmosphere. While results from in-SEM Raman (CEA, DAM) on microparticles after two years storage time point to the formation of U3O8 and a minor fraction of schoepite phase (hydrated UO3), in U L3-edge XAFS after four months storage time and U M4-edge HR-XANES after ten months storage time spectra (INE-Beamline and ACT station @ KIT synchrotron radiation source) mainly U(IV) and U(V), respectively, was observed. These results provide new insight into ageing mechanism of the microparticles after preparation. From these results important conclusions with respect to storage conditions and shelf life of the reference particles can be drawn. The first batch of pure U-oxide microparticles produced in Juelich was successfully certified regarding the isotopic composition and the U amount per particle and applied in an international laboratory exercise NUSIMEP-9.

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Shannon Potts

Papers

Chemical and structural investigations on uranium oxide-based microparticles as reference materials for analytical measurements