A subset of the Mechanistic Fuel Failure-3 (MFF-3) irradiations included U-10wt%Zr (U-10Zr) fuels that were sodium bonded to HT9 cladding with a smear density of 75% and maximum burnup of 13.5 at %. Although early PIE of U-10Zr fuels from EBR-II provided a correlation between the phase transformations and porosity via BSE-SEM imaging, there is a lack of detailed understanding of the crystal structure of phases formed, phase distribution, constituent redistribution, fuel-cladding chemical interaction (FCCI), and mechanical property degradation. Although transmission electron microscope (TEM) and three-dimensional electron backscatter diffraction (3D EBSD) experiments of the irradiated fuel via recently awarded Rapid Turnaround Experiments (18-1243 and 18-1590) will provide a detailed analysis of the phases present at different regions of the fuel, a knowledge gap still persists in understanding the microstructure-mechanical property correlations in these fuels. Additionally, there is a lack of critical assessment on the deviations in microstructure-mechanical properties between the irradiated MFF-3 fuel and unirradiated control fuel to accurately describe the alterations from irradiation. To correlate the microstructural and mechanical property changes occurring at different regions of the irradiated fuel, we propose nanoindentation measurements of major phases at various regions of the irradiated and control fuel, with a specific emphasis on the FCCI and cladding regions. For this work, we propose in-situ nanoindentation measurements using the Hysitron PI-88 nanoindenter in a Plasma Focused Ion Beam (PFIB)-SEM at IMCL along with correlative EDS and BSE information. Correlation between the proposed indentation tests and TEM/3D EBSD assessments of the fuels from recently awarded the RTE’s (18-1243 and 18-1590) will provide an understanding of the mechanical behavior of the specific phases in the FCCI and cladding regions. This can be particularly important when trying to mitigate brittle phase formation during the design of advanced reactors and materials. The proposed indentation experiments should be conducted near regions from where the TEM lamellas were produced from the RTE (18-1243) to provide a one-to-one comparison between the phases observed from the lamella and nanohardness measurements. Nanoindentation measurements on fission products (FPs) precipitates observed at various locations in the fuel is also suggested to identify the potential existence of brittle phases which could lead to fuel cracking, and eventually failure. This proposed work, combined with the synchrotron experiments at ANL, and the recently awarded RTEs for TEM and 3D EBSD/EDS assessment of fuels will also provide the first ever quantitative assessment of microstructural-mechanical property correlations in neutron irradiated U-10Zr fuels. Furthermore, the mechanical properties from these experiments (hardness and elastic moduli) could also be used to enhance the fuel performance codes for irradiated U-10Zr/HT-9 fuels.

This proposed research is directly related to the DOE-NE mission by addressing two of the four research initiatives, specifically through the development of technologies and other solutions that can improve the reliability, sustain the safety, and extend the life of current reactors and sustainable fuel cycles. These research initiatives will be addressed by providing a more comprehensive understanding of constituent redistribution, swelling, and fuel-cladding chemical interaction (FCCI), and mechanical property changes, many of which are concerns in a variety of fuels. This research links directly to the Nuclear Technology Research and Development program under the Fuel Cycle Technology initiative, as well as to the Nuclear Reactor Technologies within the Advanced Reactor Technologies, through providing insight into mechanical property changes, swelling, phase evolution, constituent re-distribution, and fuel performance in advanced fuels. Moreover, this research can be extended to establish a better understanding of FCCI and develop mitigation strategies for multiple fuel forms currently being explored within DOE-NE. This work will also provide a direct comparison to unirradiated and irradiated U-10Zr fuels fabricated in the same manner. Information acquired from previously awarded RTE’s (2017 and 2018) with TEM and 3D EBSD/EDS characterization of phases and cavities of U-10Zr fuels that have been irradiated to a moderate burnup (~ 6 at%) coupled with recent SEM and synchrotron results within this fuel will be used to complement this proposal findings. Finally, this research can be used to improve existing fuel performance codes by providing critical mechanical property data currently lacking in various cladding/fuel regions.