Electron microscopic evaluation and fission product identification of irradiated TRISO coated particles from the AGR-1 experiment: A preliminary review

Abstract

In this paper a brief summary of results from characterization of microstructures in the coating layers of selected irradiated fuel particles with burnup of 11.3% and 19.3% FIMA will be given. The main objectives of the characterization were to study irradiation effects, fuel kernel porosity, layer debonding, layer degradation or corrosion, fission-product precipitation, grain sizes, and transport of fission products from the kernels across the TRISO layers. Characterization techniques such as scanning electron microscopy, transmission electron microscopy, energy dispersive spectroscopy, and wavelength dispersive spectroscopy were used. A new approach to microscopic quantification of fissionproduct precipitates is also briefly demonstrated. Microstructural characterization focused on fission-product precipitates in the SiC-IPyC interface, the SiC layer and the fuel-buffer interlayer. The results provide significant new insights into mechanisms of fission-product transport. Although Pd-rich precipitates were identified at the SiC-IPyC interlayer, no significant SiC-layer thinning was observed for the particles investigated. Characterization of these precipitates highlighted the difficulty of measuring low concentrations of Ag in precipitates with significantly higher concentrations of Pd and U. Different approaches to resolving this problem are discussed. An initial hypothesis is provided to explain fission-product precipitate compositions and locations. No SiC phase transformations were observed and no debonding of the SiC-IPyC interlayer as a result of irradiation was observed for the samples investigated. Lessons learned from the post-irradiation examination are described and future actions are recommended.

Identification of silver and palladium in irradiated TRISO coated particles of the AGR-1 experiment

“…It was for example found in the earlier studies [13] that the micron-sized Pd-rich precipitates were visible up to approximately 15 μm from the SiC/IPyC interface.…”

“…These studies further showed significant Pd corrosion on the SiC-PyC interface. However, in the AGR-1 experiment, no significant Pd corrosion is observed [13]. The Pd and Ag concentrations used in these out-of-pile research studies, are order of magnitude larger than in the actual TRISO coated particles, which could contributed to the different corrosion behaviour observed compared to those of the AGR-1 experiment.…”

“…Although evidence of fission product precipitates were found in the SiC on SiC grain boundaries and IPyC layers, no silver was conclusively identified [13]. It was speculated that Ag would be able to substitute for Pd in a U(Ag,Pd) 2 Si 2 solid solution rather than forming a separate phase since they have identical atomic radii (both 0.144 nm for the pure elements [14]).…”

Associations of Pd, U and Ag in the SiC layer of neutron-irradiated TRISO fuel

“…TEM lamellae were prepared from the SiC layer of this particle by standard focused ion beam (FIB) techniques. More information on the particle selection process and sample preparation can be found in [5] and [9].…”

Palladium and ruthenium supported silver migration in 3C–silicon carbide

“…Apart from results from the AGR-1 experiment [5][6][7], studies of Ag release through TRISO particles mostly focussed on out of reactor simulations using bulk-or epitaxial SiC wafers. In this work we investigate the migration behaviour of a Pd-Ag combination in surrogate TRISO particles containing ZrC kernels at high temperature without neutron irradiation.…”