Microstructural Insights into Fuel Cladding Chemical Interaction in a U-20Pu-10Zr Fuel Irradiated in EBR-II

Fuel cladding chemical interaction (FCCI) is a detrimental phenomenon, limiting operational temperature and burnup in metallic nuclear fuels. Understanding the microstructure formed as a result of FCCI is critical for developing a mechanistic understanding and models for fuel behavior under irradiation. This study focused on characterizing a HT9 clad U-20Pu-10Zr-1.2Am-1.3Np metallic transmutation fuel that was irradiated in the Experimental Breeder Reactor II to a burnup of 6.15 at.% with a time-averaged peak inner cladding temperature of 540°C. Advanced characterization techniques, including transmission electron microscopy (TEM), scanning TEM with energy dispersive X-ray spectroscopy (STEM-EDS), and selected area electron diffraction (SAED), were employed to analyze phases formed within the fuel-cladding interdiffusion region. Additionally, these techniques were combined with four-dimensional scanning transmission electron microscopy (4D-STEM) to analyze the HT9 cladding microstructure. Analysis around the fuel-cladding interface revealed that Fe diffused only 1-2 µm into the fuel to form a (Fe,U)Zr₂ phase while Am, Pu, and lanthanide (Ln) fission products (Nd, Sm, Ce, La) diffused up to 1 µm into the cladding to interact with Fe. A 1-2 µm interaction zone filled the gap between fuel and cladding and primarily consisted of an Fe-Am-Pu-rich matrix with Cr-rich and Ln-rich precipitates dispersed throughout the layer. STEM-EDS combined with 4D-STEM analysis unveiled M23C6 carbide, χ-FeCrMo, and G-phase MnNiSi-rich precipitates ranging from 10-100 nm in size within the HT9 matrix near the FCCI region.