Shelf-Life of Uranium Oxide Microparticle Reference Materials and Possible Implications for the Identification of Optimal Storage Conditions

Among other tasks, the International Atomic Energy Agency (IAEA) is responsible to verify the peaceful use of nuclear material and technologies concerning the Non-Proliferation Treaty (NPT). Therefore, the IAEA Safeguards were introduced as technical measures to assure the compliance with the NPT by its member states. To secure this, IAEA officials conduct inspections of nuclear facilities all around the world and – among other measures – take swipe samples of surfaces in said facilities. Cotton swipes with collected particles are then sent to the IAEA Safeguards Analytical Service Environmental Sample Laboratory (IAEA SGAS-ESL) and its dedicated Network of Analytical Laboratories (NWAL) to analyze particle elemental and isotopic composition. The quality of these measurements is dependent on the abundance of suitable reference materials. To qualify as potential reference materials, uranium microparticles must be of a size and shape as well as an isotopic and elemental composition similar to the environmental samples. In the Forschungszentrum Jülich (FZJ), uranium microparticles are produced using a Vibrating Orifice Aerosol Generator (VOAG).<br/><br/>A core requirement for distribution of a potential reference material is to guarantee a practical shelf-life. Previous studies of structure and shape of uranium oxide microparticles demonstrated alteration process leading to the formation of uranium hydroxide like schoepite. To further investigate this alteration process, a systematic shelf-life study of VOAG-produced uranium microparticles has been launched in late 2021. In a first part of this study, the influence of different atmospheric conditions was evaluated. Uranium microparticles from the same production batch were deposited on Glass-like Carbon Disks (GCDs) and stored under different atmospheric conditions. The second part of the shelf-life study is related to the long-term stability in different solvent media. The uranium particles used in both approaches were periodically measured using Scanning Electron Microscopy (SEM) and µ-Raman Spectroscopy for structural as well as Large Geometry-Secondary Ion Mass Spectrometry (LG-SIMS) for isotopic investigation. The combination of these aforementioned methods allows to check the long-term stability of uranium microparticles in suspension and under different atmospheric conditions and therefore identify optimal storage conditions for potential reference materials. In this presentation recent results of atmospheric and suspension shelf-life studies will be discussed.