Radial microstructural evolution in low burnup fast reactor MOX fuel

Examination of irradiated fast reactor mixed-oxide (MOX) fuel was conducted to analyze microstructural behavior and defect evolution as a function of radial position. The fuel analyzed in this work was irradiated to 3.4% fissions per initial metal atom (FIMA) in the Fast Flux Test Facility. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) were used to analyze microstructural features and fission product secondary phases. A dual-beam focused ion beam (FIB)/SEM was used to prepare several samples along the fuel radius for transmission electron microscopy (TEM) defect analysis. The microstructure near the center of the fuel pellet showed separated grain boundaries accompanied by fission gas bubbles. The fuel pellet did not reach the threshold linear heating rate (LHR) needed for restructuring, formation of the columnar or equiaxed regions was not observed. Metallic fission products aggregated in pores along grain boundaries near the pellet center, but no metallic precipitates were visible in the outer region. Moreover, no appreciable amount of insoluble perovskite phase was formed. The dislocation loop density is highest near the fuel central void but decreases past the mid-point of the fuel radius likely due to the self-irradiation effects. Dislocation lines follow an inverse trend, with the lower density observed near the hottest regions and increasing in the pellet outer radius. These results indicate that the thermal gradient in MOX fuels heavily influences the localized fission product morphology and defect behavior.