Evaluating the microstructure and origin of nonmetallic inclusions in as-cast U-10Mo fuel

Abstract

Low enriched uranium alloyed with 10 wt. % molybdenum (U-10Mo) has been identified as a promising alternative to highly enriched uranium oxide dispersion fuels for use in high performance research and test reactors. Manufacturing U-10Mo alloy fuel involves several complex thermomechanical processing steps and understanding of the microstructure and its evolution throughout the various fabrication steps is critical to enable the deployment of a reliable fuel production capability. Nonmetallic inclusions are often found in U castings and may affect subsequent fuel processing steps and microstructure evolution. Yet, the origin of these inclusions is not well established. To elucidate their origin and formation mechanisms, nonmetallic inclusions in U-10Mo castings were characterized on sub-nanometer to millimeter scale. Inclusion distribution, morphology, size, and composition were determined for the metallic fuel samples. Inclusions were estimated to comprise ~ 0.4 % of the fuel (by area), were identified at both grain boundaries and grain interiors, and were found to have varying morphologies (e.g., core-shell, elongated, blocky). All inclusions were either uranium carbides or oxides, or a combination of the two (i.e., dual-phase inclusions). Analysis of inclusions via atom probe tomography revealed that carbides and oxides were hypostoichiometric, with minor amounts of additional impurity elements present (e.g., Si, H). All analyzed inclusions were found to be enriched to ~20 at. % 235U, consistent with the surrounding γ-UMo matrix and target enrichment for the low enriched U fuel, indicating that the inclusions formed during the downblending of highly enriched U metal with depleted U via the melting and casting processes.