Grain size and phase purity characterization of U₃Si₂ fuel pellets

Characterization of U₃Si₂ fresh fuel pellets is important for quality assurance and validation of the finished product. Grain size measurement methods; phase identification methods, using scanning electron microscopes equipped with energy dispersive spectroscopy, and x-ray diffraction; and phase quantification methods, via image analysis, have been developed and implemented on U₃Si₂ pellet samples. A wide variety of samples were characterized that fall into 3 fabrication periods: 1. Representative pellets from an initial irradiation experiment, 2. Small scale batches that improved upon experience from the first period, and 3. Pellets fabricated at a slightly larger scale using optimized methods to enhance phase purity. The average grain size for pellets from the first period of fabrication was between 16 and 18 μm. The typical average grain size for pellets from the second period of fabrication was between 20 and 30 μm. In the third period of fabrication samples exhibited irregular grain growth with a bimodal grain size distribution consisting of coarsened grains (>80 μm in most samples) surrounded by the typical (20–30 μm) grain structure around the surface. Phases identified in the first period uranium silicide pellets included: U₃Si₂ as the main phase composing about 80 vol %, Si rich phases (USi and U₅Si₄) composing about 13 vol %, and UO₂ composing about 5 vol %. U₃Si₂ pellet samples from the second period of fabrication had similar phases and phase quantities. Pellet batches from the third period of fabrication did not contain Si rich phases, and had between 1 and 5% UO₂: achieving U₃Si₂ phase purity between 95 vol % and 98 vol % U₃Si₂. The amount of UO₂ in sintered U₃Si₂ pellets is correlated to the length of time between U₃Si₂ powder fabrication and pellet formation. These measurements provide information necessary to optimize fabrication efforts and a baseline for future work on this fuel compound.