Characterization of Fuel Cladding Chemical Interaction on a High Burnup U-10Zr Metallic Fuel via Electron Energy Loss Spectroscopy Enhanced by Machine Learning

Abstract

Fuel cladding chemical interaction (FCCI) plays a key role in limiting the performance of metallic fuels in nuclear applications. A comprehensive analysis of chemical elements present in FCCI region is the basis for understanding the phenomena and developing potential mitigating strategies. The detection of low atomic number elements (Z < 11) and lanthanide fission products is challenging for energy dispersive x-ray spectroscopy (EDS). This work used scanning transmission electron microscopy (STEM) based electron energy loss spectroscopy (EELS) to study the distribution of carbon and lanthanides in the FCCI region of a solid U-10Zr (wt%) fuel irradiated to 13.2 at. % burnup at the Fast Flux Testing Facility (FFTF). Processing the STEM-EELS data involved three major steps: 1) enhancing the signal-to-noise ratio by denoising the STEM-EELS spectra using principal component analysis (PCA) methods; 2) identification and mapping of chemical elements with core energy loss edges; 3) microstructural phase segmentation using the K-means clustering method. STEM-EELS analysis indicated the formation of zirconium carbide, a rind-like microstructural phase, in the FCCI region between fuel and cladding. The rind appeared to remain intact at this location for the studied burnup. The study also revealed a shift in the plasmon peak between zirconium-rich region and zirconium carbide. The STEM-EELS mappings demonstrated a different distribution of Ce from other lanthanide elements, such as La, Pr, and Nd, suggesting that the effect of lanthanides in the FCCI region should be separately investigated. The use of K-means clustering method on the STEM-EELS spectra of the FCCI region revealed different phases, especially Fe-Ce and Zr-C, that concurred with the findings from STEM-EELS elemental mappings.