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.