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- Title
- Physicochemical Characterization of the SiC Barrier Layer of Irradiated TRISO Fuel
- Creator
- Seibert, Rachel
- Date
- 2018
- Description
-
Tristructural-isotropic (TRISO) nuclear fuel is designed to be an inherently safe microencapsulated fuel for use in advanced nuclear reactors...
Show moreTristructural-isotropic (TRISO) nuclear fuel is designed to be an inherently safe microencapsulated fuel for use in advanced nuclear reactors. TRISO fuel is comprised of an uranium-carbide/uranium-oxide mixed kernel, surrounded subsequently by a porous carbon buffer layer, pyrolytic carbon, silicon carbide, and pyrolytic carbon. The silicon carbide layer acts as both the structural backbone and the main barrier to non-gaseous fission product release from the kernel. During operation, fission products are known to release from intact fuel (silver) and to locally corrode the silicon carbide layer (palladium). Release of silver can pose a threat to safety and maintenance workers due to plate-out on reactor components, while palladium corrosion leads the possibility of failure of the SiC layer. An understanding of the silver release mechanism, the diffusion of palladium and fuel kernel components, and their possible correlation is necessary to understand envelopes for safe operation using these fuels.This work focuses on analysis of the reactivity of silver, palladium, and fissile inventory with silicon carbide to determine potential mechanisms and interactions. Irradiated TRISO particles and model thin film cubic silicon carbide (3C-SiC) surfaces were studied to compare and understand the reaction mechanisms of these fission products. Analysis on both systems involved X-ray Absorption Fine Structure Spectroscopy (XAFS) measurements to determine the local atomic structure of the bulk material and electron microscopy studies to observe the microstructure and fission product location. Additionally, x-ray photoelectron spectroscopy (XPS) was used to study the fundamental surface science of silver and palladium in the 3C-SiC thin films. From the comparative studies in this work, the interaction of silver, palladium, and fuel kernel products has been observed. Palladium silicides are preferentially formed, but silver remains metallic and diffuses through both bulk SiC, through material defects, and through grain boundary diffusion. Uranium preferentially forms carbides with increasing temperature. Plutonium primarily forms silicides, but with short carbon bonds. All fission products and fissile inventory were observed to be segregated along grain boundaries in the SiC. This work did not measure a single release mechanism, but suggests multiple mechanisms work together simultaneously. It also presents the first evidence of uranium carbide formation at elevated temperatures and of silver as a metal in irradiated TRISO fuel, to the best of the author's knowledge.
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