"Analyzing Microstructural Evolution Resulting from Varied Irradiation Conditions in Redistribution Regions of Historical Uranium-10wt% Zirconium Fuel" 

Morgan Smith, MSE PhD Candidate 

Advisor: Professor Maria Okuniewski

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ABSTRACT

Uranium-zirconium fuels are used in fast reactors for their higher melting temperatures, excellent thermal conductivity, and exceptional performance at high burnups. However, during fission, these fuels produce gaseous and solid fission products, leading to fuel swelling that degrades fuel thermomechanical properties as well as porosity formation from irradiation damage and diffusion. The pores generated during in-pile exposure can be stabilized with gaseous and solid fission products, but the behavior of lanthanides, major constituents of solid fission products, in U-Zr fuels is not fully understood, especially in their relation to the morphology and density of pores formed. Furthermore, while swelling has been studied via fuel pin axial growth in Experimental Breeder Reactor-II (EBR-II) and the Fast Flux Test Facility (FFTF), there are few works investigating how swelling rates evolve under various irradiation conditions. This study examines porosity distribution in historical and recent experimental U-10wt%Zr cross sections from EBR-II and FFTF to understand pore morphology evolution in constituent redistribution regions as irradiation conditions are varied. Automated image analysis and stereological principles were applied to analyze micrographs. Scanning electron microscopy (SEM) and electron dispersive spectroscopy (EDS) were used to characterize the lanthanide concentration at various localities in the fuel. Micro-computed tomography and SEM images allowed for pore morphology to be characterized in multiple dimensions. Results showed porosity, pore size, redistribution region size, and lanthanide concentration evolution across different burnups and temperatures, offering insights into microstructural changes and fuel performance, essential for developing performance codes on swelling.