Characterization of local anisotropic swelling and fragmentation at the top of metallic nuclear fuel slugs and its significance on fuel performance
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Authors
Fay, Jake
Issue Date
2025-12
Type
Electronic thesis
Thesis
Thesis
Language
en_US
Keywords
Nuclear engineering
Alternative Title
Abstract
U-Zr and U-Pu-Zr metallic fuel alloys have been widely studied as a possible fuel type for sodium fast reactors and other liquid metal cooled reactor concepts. Major experiments such as the Experimental Breeder Reactor II (EBR-II) and Fast Flux Test Facility (FFTF) subjected metallic fuels to extensive irradiation campaigns to optimize metallic fuel designs and compositions. During the operation of these experiments radiography of metallic fuel pins showed the formation of a highly porous, irregularly shaped structure at the top of the fuel pins. This structure has been given the name metallic fuel free-surface swelling and will be referred to as “MFFS” for the remainder of this dissertation. Destructive post irradiation examinations (PIE) of the top of an irradiated EBR-II pin showed that MFFS is predominantly comprised of fuel elements in similar proportions to as cast fuel. The presence of fissile atoms in the MFFS structure could lead to unexpected neutronics behavior or even safety concerns if the MFFS structure becomes detached under a reactor transient. The focus of this dissertation has been to further investigate the MFFS structure and gain insights into its formation mechanism through a combination of reviewing legacy data from EBR-II, separate effects studies performed at RPI, and new PIE performed at Idaho National Laboratory (INL). Chapter 1 of this report gives a thorough background on metallic fuels, their swelling behavior under irradiation, and previous work that has been done to investigate the MFFS structure. Chapter 2 reviews legacy radiographs and operating data from EBR-II to correlate MFFS content with key reactor operating conditions. Chapters 3 and 4 compare 3D characterizations performed on cubes taken from the MFFS and bulk fuel regions. Chapter 5 discusses macro-scale characterizations and optical microscopy performed on newly sectioned MFFS cross section. Chapter 6 discusses microscale characterizations of low burnup U-10Zr fuel samples under low irradiation temperatures. Finally, Chapter 7 discusses separate effect creep testing performed on porous uranium and U-10Zr samples at elevated temperature. The completed work currently suggests the MFFS forms due to there being high a high proportion α-phase uranium at the top of metallic fuel. α-phase uranium undergoes highly anisotropic swelling which causes it to form jagged highly porous structures in the absence of mechanical constraint at the top of the fuel slug. Fuel creep behavior at the top of the fuel slug also likely contributes to MFFS formation.
Description
December2025
School of Engineering
School of Engineering
Full Citation
Publisher
Rensselaer Polytechnic Institute, Troy, NY