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

Abstract

Particle analysis is one of the key-techniques used in the field of nuclear safeguards. Beyond traditional uranium isotopy measurement, other methodologies are implemented to better characterize nuclear materials. Among them, age dating at the particle scale enables to determine the time elapsed since the last chemical step of separation/purification or enrichment. The 230Th-234U couple being one of the most common radiochronometer, there is a need for morphology and size controlled uranium-thorium mixed oxides particles that could be used as reference materials during isotopic measurements. With this aim, uranium-thorium mixed oxide microspheres were synthesized using a wet chemistry route. The hydrothermal conversion of aspartate precursors at T = 433 K led to mixed dioxide micro-particles with controlled spherical morphology and size, up to 5 mol% in thorium. In order to remove impurities, densify the micro-particles, and control the chemical form of the final compounds, heat treatments were performed under various atmospheres. Nearly stoichiometric (U,Th)O2 dioxides were obtained under reducing conditions (Ar-4%H2) while U3O8-based samples were formed under air, with thorium incorporated in the structure up to 2 mol%. Last, the homogeneity of the cation distribution in the samples was evaluated by various methods, including PERALS α-scintillation counting, as well as X-EDS and LG-SIMS analyzes of individual particles, leading to consistent results. Particularly, the relative external reproducibility (2σ) of the 232Th+/238U+ ion ratios measured at the particle scale remained below 10%, paving the way to use these mixed oxide particles in the field of nuclear safeguards.