Electron microscopy analysis of high burnup commercial fuel

Modern electron microscopy analysis can provide detailed insight into the microstructure and chemistry of high burnup UO₂ fuel. This more complete understanding of fuel evolution and performance in reactor can provide critical insight necessary to validate fuel performance models and optimize reactor operations. Preliminary results from the investigation of a polished cross-section of high burnup UO₂ fuel (seven cycles) from the H.B. Robinson pressurized water reactor will be discussed. The analysis focuses on scanning electron microscopy (SEM) of the complete cross-section, while scanning transmission electron microscopy (S/TEM) was performed on targeted features identified by SEM where specimens were fabricated by traditional focused ion beam lift-out techniques. Analysis of the complete cross-section allowed for investigation of the microstructural variations from centerline to fuel/clad interface, pellet/pellet interface, and cladding. Secondary electron imaging and backscatter electron imaging was performed to identify variations in the distribution of fission product secondary phases and bubbles/voids. The analysis of the pellet/pellet interface is a unique feature of this sample where initial analysis shows segregation of fission product phases at the pellet/pellet interface and also indicates an interface microstructure lacking of bubbles and voids. X-ray spectrum imaging was performed by both SEM and STEM techniques to identify the composition of the secondary phases present in the fuel. Multivariate statistical analysis (MVSA) of the X-ray spectrum images has been applied to improve the sensitivity of the analysis and highlight minor variations in the fuel chemistry and secondary phases comprised of fission product as well as the distribution of sub-surface Xe gas bubbles in the high burnup structure.