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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.

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Analysis Key information pertaining to fission product release Capsule components Cumulative inventory of fission products released from all fuel compacts in a capsule during irradiation Fuel compact gamma scanning Retained inventory of fission products in each individual fuel compact Deconsolidationleach-burn-leach Inventory of fission products retained in the compacts outside of the SiC layers Particle gamma counting Retained inventory of fission products in the particles 2.2.1. Analysis of irradiation capsule components for deposited fission products The major components of the irradiation capsules-including the metal shells and structural components, the graphite fuel holders, and graphite spacers at the ends of the capsules-were analyzed at INL to determine the cumulative inventories of fission products released from the compacts during the irradiation. As there were 12 compacts in each irradiation capsule, the data provide information on the total inventory of fission products released from all 12 fuel compacts; release from each compact cannot be derived from these data. The metal capsule shells were leached in acid to remove deposits. Analysis of the leach solutions included gamma spectrometry for gamma-emitting fission products, Sr separation and gas flow proportional counting for 90