Fission product swelling is one of the main failure modes in metallic fuels. At high burnup the pores can form interconnected networks which can cause an increase in gaseous diffusion, increasing interaction with cladding, and degrading mechanical and thermal stability of the fuel. To understand the behavior of pore morphology, characterization of the microstructure in three dimensions is necessary. There are two main techniques for this: x-ray micro-computed tomography (µ-CT) and serial sectioning via a focused ion beam/scanning electron microscope (FIB/SEM). By analyzing three samples that were previously measured utilizing synchrotron µ-CT through serial sectioning, a unique opportunity is presented to correlate the microstructures revealed in both techniques. Utilizing this multimodal technique approach in a serial fashion to determine the microstructure of the samples, can reveal a more in-depth understanding of the material behavior and validate the structures seen in both techniques. µ-CT has a higher accuracy of spatial geometry of features due to the lack of curtaining, ion damage, and local heating possible in serial sectioning, yet information on grain boundaries, crystal structure, and microchemistry of the surrounding matrix can only be obtained through serial sectioning of the material. Moreover, µ-CT typically has a spatial resolution of ~1 micron, whereas features that are submicron are detectable using the FIB/SEM technique. Therefore, by comparing the results from both techniques, a more comprehensive understanding of the pore morphology can be obtained, which will allow for more accurate microstructures in high burnup pore morphology models.
Following high burnup, uranium-molybdenum (U-Mo) nuclear fuel undergoes grain refinement producing smaller and more numerous grains. This increase in grain boundaries should increase the diffusivity of the fission gases through the material exhibiting behavior that is seen in higher operating temperature fuels of various types. To test this hypothesis, it is necessary to look at the morphology of a high burnup and high operating temperature fuel such as U-Zr. To minimize the multiphase effects on the pore behavior, an interior sample of irradiated U-Zr will be analyzed with serial sectioning and compared to previous synchrotron µ-CT results. Then compared to the microstructure seen in post-recrystallized U-Mo. Serial sectioning the FIB lift outs and conducting secondary electron (SE), backscatter electrons (BSE) scans on each layer in the U-Mo (two specimens) and U-Zr (one specimen) fuels will provide detailed information on the morphology of the features which can be compared to previous tomography results. Energy dispersive X-ray spectroscopy (EDS) scans will give insight on chemical distributions of the fuel and fission products, as well as give confirmation on phenomenon that are suggested in the tomography results. Electron backscatter diffraction (EBSD) will provide grain boundary density information, which can influence the gaseous fission product diffusion, therefore affecting both growth rate and nucleation rate of pores. By utilizing multimodal techniques to analyze phenomena seen in these samples in-depth information on the high burnup fuel behavior will be elucidated. Another goal of the proposed work is to create a more global model of pore morphology evolution in metallic based fuels.