Abstract
Metallic fuels, including U-Zr and alloyed derivatives, are promising candidates for advanced fast reactor applications due to their high thermal conductivity, high burnup compatibility, inherent passive reactor safety, and favorable neutronic properties. However, their performance is limited by multiple factors, including but not limited to, void swelling, fuel cladding chemical interaction (FCCI) and complex distribution of fission products under irradiation. This dissertation presents a comprehensive, correlative characterization of secondary phases and fission products present in irradiated metallic fuel systems using state-of-the-art techniques such as transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) coupled with electron energy loss spectroscopy (EELS), atom probe tomography (APT), and focused ion beam time-of-flight Secondary Ion Mass Spectrometry (FIB-ToF-SIMS).In irradiated U-10Zr (wt.%) fuel from the Fast Flux Test Facility (FFTF), STEM-EELS enabled mapping of low atomic number elements (Z<11) and lanthanide (Ln) fission products in the FCCI region. Principal component analysis (PCA) and K-means clustering revealed the formation of a Zr-C-rich rind at the fuel cladding interface, along with distinct Ce distribution compared to other lanthanides. FIB-ToF-SIMS analysis detected open and closed fission gas pores across various radial locations in the irradiated U-10Zr fuel. The presence of lanthanide fission products (Ln-FPs) within these pores, coupled with the detection of Cs and Na along the pore edges, suggests a liquid-mediated transport mechanism. Complementary TEM and APT studies of annular U-10Zr fuel, irradiated at the Advanced Test Reactor (ATR), revealed nanoscale U and Zr clustering and fission product segregation in the inner fuel region. Zr-rich clusters exhibited enhanced fission product concentrations relative to U-rich clusters, suggesting co-precipitation during post-irradiation cooling.
Investigations of a pseudo-binary U-10MTZ (10MTZ: 5Mo-4.3Ti-0.7Zr, wt.%) fuel irradiated at ATR revealed an FCCI region dominated by U-Fe interdiffusion with minimal involvement of lanthanides. The cladding-side FCCI region consisted primarily of the (U,Zr)(Fe,Cr)₂ intermetallic phase, within which isolated pores and microcracks were observed. In addition, a previously unreported Fe–Cr–Te intermetallic phase containing Nd and Ba was identified in this region, indicating an association between Te and cladding constituents. APT further revealed the presence of trace fission products such as Kr, Te, Sn, Gd, and Dy near isolated pores in this region.
Together, these multi-scale, correlative analyses elucidate the mechanisms driving fission product distribution, secondary phase evolution, and interfacial stability in irradiated metallic fuels. The findings provide a foundation for designing advanced metallic fuels with improved FCCI resistance and irradiation performance.