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HIGH TEMPERATURE DEGRADATION OF SILICON CARBIDE (SIC) BY METALLIC FISSION PRODUCTS (PALLADIUM, SILVER) AND OXIDATION
[Thesis]. Manchester, UK: The University of Manchester; 2018.
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Abstract
One critical safety issue with tri-structure isotropic (TRISO) fuel of high temperature reactors (HTRs) is the high release fraction of the fission product (FP) silver (Ag) through the intact silicon carbide (SiC) coating. Aside from Ag release, chemical attack of another fission product, pallidum (Pd), on the SiC coating is of particular interest, as Pd has a direct impact on the migration of other fission products, including Ag, through the SiC layer, the major diffusion barrier in coated fuel. A number of studies on Ag migration in SiC have been performed in both, in-pile and out-pile studies, with various mechanisms proposed. However, Ag release does not appear to be simply controlled by one existing diffusion mechanism. To have a thorough understanding of how the metallic fission products Pd and Ag behave and migrate in SiC, reaction couples Ag-SiC, Pd-SiC and PdAg-SiC were annealed from 1100°C to 1400°C to simulate the FPs-SiC interaction at the regular operation temperature of TRISO particles. Both thermodynamics and experimental work suggested that the chemical composition of FPs directly influences their migration in SiC. Liquid Ag (Si) alloy dissolved SiC in a limited way but this dissolution cannot effectively help Ag migrate through SiC below 1400°C due to the insufficient driving force for activating “reaction-recrystallization†migration. Pd penetrated easily into SiC leaving SiC severely dissolved. FP clusters containing Pd, Ag and possibly Si can penetrate SiC with SiC remaining intact via reaction-recrystallization when the composition of clusters (atomic Pd:Ag) was in a specific range. Here, it is proposed to modify the metallic cluster’s composition by adding extra Si to relieve the chemical dissolution of SiC. Initial experimental work confirmed the potential of this proposal, which could be useful in mitigating Ag release in TRISO fuel. Oxidation experiments were carried out on the SiC coating of three compositions (stoichiometric SiC and coatings with co-deposited Si or C) in both air (1200°C, 1600°C) and steam (1200°C). Silica morphology and growth kinetics suggested that the SiC coating with co-deposited Si oxidized faster than stoichiometric or C co-deposited SiC coating in air, but a slower oxidation in the steam was observed. In addition, this work suggested that production of CO gaseous group at amorphous silica/crystalline silica interface might be responsible for initial pores birth in silica, which had a great impact on silica growth kinetics afterwards, especially in steam. This interpreted the low oxidation rate of Si co-deposited SiC coating in steam due to the limited contribution of CO gases from Si, compared with other two coatings.