Synthesis and Stability Assessment of Uranium Microparticles: Providing Reference Materials for Nuclear Verification Purposes

To verify the commitment of States signatory to the Treaty on the Non-Proliferation of Nuclear Weapons(NPT), the International Atomic Energy Agency (IAEA) conducts inspections in nuclear facilities. During such inspections, environmental swipe samples are collected to verify the absence of undeclared activities. The collected samples are shipped to dedicated laboratories in the IAEA Network of Analytical Laboratories (NWAL), where individual microparticles containing fissile material collected on such swipe samples are investigated with high accuracy to determine the isotopic composition. Due to the improved capabilities to detect and analyze individual microparticles, especially since the introduction of large-geometry secondary ion mass spectrometers (LG-SIMS), there is a great need for dedicated particulate samples which can be used for various quality control purposes. Within this work, a method was established to produce micrometer-sized particles with a monodisperse particle size distribution. The particles consist of a known amount of uranium oxide with a known uranium isotopic composition, and are intended to be certified as certified reference material (CRM). In order to produce such microparticles, a spray-pyrolysis synthesis method based on the generation of a monodisperse aerosol using a vibrating orifice aerosol generator (VOAG) was used. The method is based on the generation of an aerosol from a solution with a known uranium isotopic composition, after which the generated droplets are heated. During heating, the solvent evaporates leading to theprecipitation of the dissolved uranium into spherical shaped particles. The precipitated particles are thermally decomposed leading to the formation of uranium oxide particles. To investigate the particle formation and subsequent thermal decomposition, aerosols were produced from uranyl chloride, uranyl acetate and uranyl nitrate precursor solutions. The generated droplets from the different precursors were heated up to 600 $^{∘}$C, after which the produced particles were investigated by combined scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopic (EDX) studies, which provided insight into the particle formation process. Particles produced from uranyl chloride provided detailed information on the various particle formation and decomposition stages, although the final particles had highly irregular particle shapes and clear voids were observed. Particles produced from both uranyl acetate and uranyl nitrate resulted in spherical shapes with monodisperse particle size distributions, although the morphology of the produced particles was found to be strongly influenced by various factors during particle formation, such as the temperature and anion to cation ratio of the precursor solution