Investigation of Constituent Redistribution in a 12 at% Burn-up Annular U-10Zr Fuel using EPMA and TEM

Uranium-zirconium (U-Zr) fuels are promising candidates for next generation fast reactors, featuring high thermal conductivities, high fissile and fertile densities, and compatibility with sodium coolant and HT-9 cladding. Swelling during irradiation of U-Zr fuels as a result of fission gas release generates fuel-cladding mechanical interactions (FCMI) and enhances fuel-cladding chemical interactions (FCCI) through fuel-cladding contact, both of which can significantly weaken the cladding at higher burn-ups. To mitigate these concerns, a new design of U-Zr fuels is under development with an as-cast annulus to allow for fuel swelling inwards into the annulus rather than outwards towards the cladding. Annular U-Zr fuels have the capacity to reach extremely high burn-ups up to 30 - 40 at%, in accordance with DOE-NE goals to increase the economic viability and safety of fuels, with a lower risk of cladding failure from FCMI and FCCI. To commercially qualify annular U-Zr fuels, experimental data on fuel performance phenomena is necessary to influence fuel performance model and simulation parameters to prove the safety and efficiency of these fuels. Constituent redistribution, the phenomenon in which fuel elements preferentially migrate along chemical and temperature gradients, influences important fuel behaviors such as porosity and thermal conductivity. Therefore, it is necessary to fully understand constituent redistribution in order to fully describe these fuels in models and to characterize their efficiency and safety. Previously, constituent redistribution has been analyzed in U-Zr fuels using methods such as energy-dispersive spectroscopy (EDS) or visual inspection with optical microscopy. These methods, while useful, lack quantitative analysis from fuel element peak overlaps for EDS. Additionally, the identification of phases from EDS or optical micrographs is largely based on equilibrium phase diagrams, which may not be valid during operating conditions in a reactor. Electron probe microanalyzer (EPMA) is similar to EDS in gathering elemental concentration data. However, EPMA data provides quantitative compositional data due to the presence of elemental standards and higher energy resolution than EDS, reducing peak overlaps. Transmission electron microscopy (TEM) with selected area electron diffraction (SAED) allows for the identification of individual phases in a sample through indexing of diffraction patterns. Combined with EPMA, SAED analysis can give a better understanding of the phase evolution and quantitative information about constituent redistribution in these samples that cannot be gathered from EDS or optical microscopy alone. Experimental data on constituent redistribution in annular fuels is lacking in current literature, with most studies concerned with quantification of porosity or the use of EDS for elemental analysis. The experimental data that has been published indicates significant differences in the number, size, and composition of constituent redistribution zones between annular and solid U-Zr fuels. To accurately model and predict fuel behavior during irradiation, more experimental data on annular U-Zr fuels is needed to better understand the fundamentals of their fuel performance phenomena. This work focuses on the phase identification and quantitative elemental concentration in an annular, U-10wt%Zr irradiated to 12 at% in the Advanced Test Reactor (ATR). Quantitative elemental analysis was completed using EPMA to identify constituent redistribution zones and their elemental concentrations. TEM with SAED was used to index individual phases in each constituent redistribution zone. Using these techniques, two constituent redistribution zones were identified in this sample: an inner Zr-rich and U-depleted zone and an outer Zr-depleted and U-rich zone. This work will primarily focus on the identification of the phases in each constituent redistribution zone and their elemental concentrations as well as compare them to solid U-10Zr designs to provide further insight into the underlying causes behind differences in fuel behaviors between U-Zr geometries. This work will help provide much needed data to better understand constituent redistribution in annular U-Zr fuels and the mechanisms that govern this phenomenon. Additionally, the phase information gathered with this sample will have important implications for updating equilibrium phase diagrams for applicability under irradiation conditions. Finally, the quantitative elemental concentrations and phase identification gathered from this work will be used to guide BISON model parameters to further fuel qualification efforts.