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

• Published in: Journal of nuclear materials Vol. 460; pp. 97 - 106
• Main Authors: Lillo, T.M., van Rooyen, I.J.
• Format: Journal Article
• Language: English
• Published: United States Elsevier B.V 01-05-2015; Elsevier
• Subjects: AGR; COATED FUEL PARTICLES; EDS; FISSION PRODUCTS; FUEL; Fuels; GRAIN BOUNDARIES; IDAHO NATIONAL LABORATORY; IRRADIATION; LAYERS; Materials Characterization; NANOPARTICLES; NEUTRONS; NUCLEAR FUEL CYCLE AND FUEL MATERIALS; NUCLEAR FUELS; PALLADIUM; Precipitates; PRECIPITATION; Silicon carbide; SILICON CARBIDES; SILVER; STEM; TEMPERATURE RANGE 1000-4000 K; TRANSMISSION ELECTRON MICROSCOPY; TRISO; URANIUM; USA, Idaho
• ISSN: 0022-3115; 1873-4820
• DOI: 10.1016/j.jnucmat.2015.02.010

•Fission products in the SiC layer of a neutron-irradiated TRISO particle were analyzed.•Pd, Ag and U found in grain boundary, matrix and triple junction precipitates.•U never found alone or with Ag, only associated with Pd.•Not all boundaries and triple junctions had fission products.•Grain boundary crystallographic parameters likely a factor in migration behavior.
Knowledge of the associations and composition of fission products in the neutron irradiated SiC layer of high-temperature gas reactor TRISO fuel is important to the understanding of various aspects of fuel performance that presently are not well understood. Recently, advanced characterization techniques have been used to examine fuel particles from the Idaho National Laboratory’s AGR-1 experiment. Nano-sized Ag and Pd precipitates were previously identified in grain boundaries and triple points in the SiC layer of irradiated TRISO nuclear fuel. Continuation of this initial research is reported in this paper and consists of the characterization of a relatively large number of nano-sized precipitates in three areas of the SiC layer of a single irradiated TRISO nuclear fuel particle using standardless EDS analysis on focused ion beam-prepared transmission electron microscopy samples. Composition and distribution analyses of these precipitates, which were located on grain boundaries, triple junctions and intragranular precipitates, revealed low levels, generally <10 atomic %, of palladium, silver and/or uranium with palladium being the most common element found. Palladium by itself, or associated with either silver or uranium, was found throughout the SiC layer. A small number of precipitates on grain boundaries and triple junctions were found to contain only silver or silver in association with palladium while uranium was always associated with palladium but never found by itself or in association with silver. Intergranular precipitates containing uranium were found to have migrated ∼23μm along a radial direction through the 35μm thick SiC coating during the AGR-1 experiment while silver-containing intergranular precipitates were found at depths up to ∼24μm in the SiC layer. Also, Pd-rich, nano-precipitates (∼10nm in diameter), without evidence for the presence of either Ag or U, were revealed in intragranular regions throughout the SiC layer. Because not all grain boundaries and triple junctions contained precipitates with fission products and/or uranium, along with the differences in migration behavior between Pd, Ag and U, it was concluded that crystallographic grain boundary and triple junction parameters likely influence migration behavior.