Tri-structural isotropic (TRISO) particles are the fuel of choice for very-high temperature reactor technology. At the core, these spherical particles consist of an inner fuel kernel of UO2, UC2 or a combination of both, which is coated radially outward by successive layers of low density pyrolytic graphite, an inner high density pyrolytic graphite layer, silicon carbide (SiC) and an outer high density pyrolytic graphite layer. SiC is the main diffusion barrier against the release of fission products due to its hardness and high melting point. Nonetheless, irradiation testing of TRISO particles indicates that trace amounts of metallic fission products, such as Ag (which as a ~41 day half-life), diffuse through SiC. Competing theoretical and experimental observations that indicate that Ag diffuses through the bulk and through grain boundaries. Diffusion through grain boundaries is usually amplified due to the formation of large grains upon crystallization by heating. This leads to the formation of triangular micro pits in thin films, suggesting that a 3D version of this form of crystallization could facilitate even more the release of Ag. Previously XAFS and XPS have shown that Ag remains metallic in a SiC matrix, indicating that bulk diffusion is perhaps interstitial rather than substitutional.<br/> <br/>In this talk I will discuss the mobility of Ag films enclosedon layers of 3C-SiC by analyzing SEM images taken before and after annealing. Image analysis supported by surface spectroscopic and crystallographic techniques are used in order to estimate the release of Ag through SiC for Ag interlayers at various thicknesses and annealing temperatures. Preliminary analysis by SEM/EDS show that upon annealing Ag escapes through the SiC surface by three different mechanisms, which roughly in chronological are: 1) escape through triangular pits and grain boundaries in SiC; 2) agglomeration and formation ofsub-surface blisters which eventually rupture; 3) diffusion through the bulk of SiC when the trapped amounts of Ag are insufficient to form large blisters. During the third mechanism, we still observed sub-surface mobility in the form of dendritic paths. When the starting amount of Ag is reduced sub-surface diffusion is limited and the release of Ag is dominated by the escape through the triangular pits of SiC and bulk diffusion.<br/>

Imaging the mobility of Ag films encapsulated in 3C-SiC as a function of annealing temperature

251st National Meeting (2016)

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