Mechanistic Fuel Failure-3 (MFF-3) consists of U-10wt%Zr fuels that are sodium bonded to HT9 cladding (6.86 mm outer diameter, 6.30 mm inner diameter) with a smear density of 75% and maximum burnup of 13.5 at %. Although early PIE of U-10wt%Zr fuels from EBR-II cores provide a suggestive correlation between phase transformations and porosity via BSE SEM imaging, there is a lack of detailed analysis on the crystal structure of phases formed and their effects on pore morphology. Several postulations regarding the effects of a-U, ß-U, y-(U, Zr) and d-UZr2 phase transformations have been made. The current hypothesis is that the fission gas accumulation and phase transformations from a temperature gradient cause the formation of isotropic pores near the center of the fuel (high temperature, isotropic y phase), while intermediate/peripheral regions at lower temperatures result in the formation of anisotropic pores along grain boundaries. However, there is a serious lack of detailed analysis (e.g. TEM) on the crystal structures of the phases formed; fission products that complex existing phases; and; inter-diffusion phases formed at FCCI regions. Additionally, there is lack of critical assessment on the deviations in microstructure between irradiated MFF-3 and unirradiated control fuel to accurately describe the phase evolution from irradiation.

Transmission electron microscopy is essential for the critical assessment of microchemical differences and the crystal structure of the phases evolved after irradiation at the center, intermediate and periphery regions of the fuel. Information acquired from TEM characterization techniques will provide a detailed analysis of microstructural phase, porosity evolution and fuel constituent redistribution in U-10Zr fuels that have been irradiated to a high burnup (5.7 at%). Ideally, preparation of 8 lamellas (2 specimens per region: A, B and C from the irradiated fuel, and 2 from the control fuel) by Focused Ion Beam (FIB) is preferred; however, the uncertainty of funds and time intervals available for preparation suggests the fabrication of 5 lamellas (1 specimen per region: A, B and C from the irradiated fuel and 2 from the control fuel) to be analyzed in the FEI Titan Transmission Electron Microscope (TEM). The FIB at EML or IMCL at the Materials and Fuels Complex at INL will be used for preparing TEM lamellas. BSE SEM images and confirmatory EDS mapping/lines scans of the 3 regions of interest would be important to correlate to the TEM and synchrotron data. Additionally, a total 3 cubes (~50x50x100um) [2 specimens from region A and C of irradiated fuel, and 1 from the control fuel] will be prepared via FIB for synchrotron based experiments at the Advanced Photon Source for 3D microstructural characterization in 2018. HRTEM, BFTEM, DFTEM, selected area diffraction, and microchemistry characterization of phases on irradiated and control U-10Zr fuels will not only provide a unique insight on phase transformations, FCCI, and, pore morphology evolution, but will also provide the first ever correlation of microstructural changes in fuels before and after irradiation.

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), 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 FCCI development, swelling, phase evolution, 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 provide a direct comparison to unirradiated and irradiated U-10Zr fuels fabricated in the same manner. Information acquired from TEM characterization techniques will provide the much needed detailed analysis of microstructural phase and porosity evolution of U-10Zr fuels that have been irradiated to a moderate burnup (~ 6 at%). The recent SEM and synchrotron results within this fuel will be used to complement this proposal’s findings. Finally, this research can be used to improve existing fuel performance codes.