Microstructural characterization of as-fabricated and irradiated U-Mo fuel using SEM/EBSD

Electron Backscattered Diffraction (EBSD) is an effective technique for revealing many details about the microstructure of materials, e.g. crystallographic orientation, grain size, grain boundary properties, texture, intragranular misorientation, and subgrain formation. Since these details are of interest for improving the irradiation performance understanding of any irradiated nuclear fuel (e.g. swelling behavior), this technique has been applied successfully on U-7 wt% Mo before and after irradiation. This fuel is a high-density, low-enriched uranium fuel currently being developed for application in research and test reactors. Based on the results of this characterization, it was found that when as-fabricated U-7 wt% Mo is irradiated to around 5.3 × 10²¹ fissions/cm³ the original large grains (diameter ∼4 μm) are subdivided into much smaller grains (diameter ∼0.3 μm) and most of these subdivided grain boundaries are low angle boundaries. The EBSD analysis suggests that the grain subdivision in the irradiated U-7 wt% Mo was driven by polygonization, not recrystallization as defined by classic metallurgy as a result of heavy cold work followed by heat treatment.