Abstract
This study investigates the chemical durability of uranium oxide microparticles (UO$_{2+x}$ and U$_3$O$_8$ ), as potential reference materials for nuclear safeguards. To optimize long-term preservation, the particles were exposed to three different storage media: dilute nitric acid (10$^{-2}$ mol.L$^{-1}$HNO$_3$ ), deionised water, and ethanol. Dissolution rates in nitric acid (~5 × 10$^{-4}$ g.m$^{-2}$ .d$^{-1}$ ) were similar to those of bulk uranium oxides, but UO$_{2+x}$ particles experienced greater absolute leaching due to their higher specific surface area, resulting in noticeable morphological changes. In distilled water, rapid precipitation of secondary phases, such as schoepite and studtite, transformed particle morphology into aggregated platelets. In contrast, ethanol preserved both particle shape and structural integrity over eight months, ensuring stability for isotopic analysis using Large Geometry Secondary Ion Mass Spectrometry (LG-SIMS). These results suggest ethanol as the most effective storage medium, especially under anhydrous conditions. While both UO$_{2+x}$ and U$_3$O$_8$ exhibited comparable durability, their reactivity was primarily influenced by microstructural differences. This study highlights the importance of selecting appropriate storage conditions based on fabrication methods and particle characteristics to maintain uranium oxide reference materials for nuclear safeguards and ensure their long-term reliability in quality control applications.