Characterization of micro-burnup treat irradiated U-22.5 at.% Zr and U-52.8 at.% Zr foils by transmission electron microscopy and X-ray diffraction

The uranium-zirconium (U-Zr) fuel system has been studied for decades, yet there remains a data gap of micro-burnup (BU) studies where the microstructure undergoes an initial transformation from as-fabricated to the true starting microstructure present in reactor. This study focusses on two U-Zr specimens, U-(22.5 and 52.8) at.% Zr, irradiated to <0.001 at.% BU (∼4.33E+15 F/cc) at 660 °C in the Transient Reactor Test Facility (TREAT). The samples were rapidly gas quenched (4–5 °C/min) to maintain the in-pile structure for post irradiation characterization and modeling input. The bulk features of the U-22.5 at.% Zr specimen were retained, suggesting an impact (beyond crystallographic texture) of the as-fabricated system. It was also found that Zr solubility in α-U increases to ∼18 at.%Zr at 660 °C, and subsequently influences phase fractions and grain boundary density. The U-52.8 at.% Zr alloy fully transformed into an equiaxed granular microstructure immediately upon reactor insertion, implying single phase γ-U-Zr at 660 °C. This work also identifies the retention of a face-centered-cubic Zr rind on the surface of the sample, suggesting that the mechanism of Zr-rind decomposition is not a trivial phase transformation at temperature, but rather likely a diffusion-based process.