Abstract
Fuel-cladding chemical interaction (FCCI) is an important phenomenon in metallic nuclear fuels, as it directly influences fuel performance, cladding integrity, and overall reactor safety. A detailed understanding of FCCI mechanisms and the role of fission products is essential for designing and developing advanced fuel systems with improved irradiation performance. In this study, we investigated the FCCI region on the cladding side of HT9 ferritic-martensitic steel in contact with U-10MTZ fuel (10MTZ = 5Mo-4.3Ti-0.7Zr, wt%), irradiated in the Advanced Test Reactor to 2.2 at.% burnup. High resolution characterization techniques, including transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), and atom probe tomography (APT), were employed to examine microstructural evolution and fission product distributions in the FCCI region. TEM revealed Ti-rich precipitates along grain boundaries of the cubic (U,Zr)(Fe,Cr)₂ phase. EELS detected Ba and lanthanide fission products (Ln-FPs) within isolated pores. APT identified a wide range of fission products, including Kr, Te, Gd, and Dy, as well as a distinct band enriched in Zr, Ti, C, and Sn adjacent to a pore. APT analyses of the HT9 cladding region detected neither carbon nor elemental clustering. These findings provide new insights into the spatial distribution and chemical behavior of fission products and their influence on microstructural evolution under irradiation, establishing a benchmark for future studies on FCCI mechanisms in irradiated U-10MTZ metallic fuels.