Abstract
Fuel cladding chemical interaction (FCCI) is one of the limiting factors for metallic fuels in nuclear application. A comprehensive analysis of chemical elements in FCCI 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 fast flux testing facility. Processing the EELS data involved three major steps: 1) enhancing the signal to noise ratio by denoising the spectrum using principal component analysis methods; 2) identification of chemical elements with core energy loss edges; 3) microstructural phase segmentation based on raw EELS spectrums using the K-means clustering method. EELS analysis revealed the formation of zirconium carbide, a rind-like microstructural phase in FCCI region between fuel and cladding. This rind appeared to be intact at this burnup. The study also revealed the shift of plasmon peak between zirconium and zirconium carbide. The EELS mapping also indicated a different distribution of Ce from other lanthanide elements, such as La, Pr, and Nd, suggesting the lanthanides should be separately investigated. The use of the K-means clustering method on the EELS data of FCCI region revealed different phases, especially Fe-Ce and Zr-C, that concurred with the findings from EDS and STEM-EELS elemental mappings.
•Presence of carbon in the Zr-rich rind was observed.•The Ln FPs were found to be well distributed in the EEL spectrum of the FCCI region.•Plasmon peak map indicated energy shift due to the formation of carbide in the rind.•The use of machine learning algorithm, K-means clustering, on EELS data automated multiple clusters representing multiple phases like Fe-Ce and Zr-C.