Advanced electron microscopic techniques applied to the characterization of irradiation effects and fission product identification of irradiated TRISO coated particles from the AGR-1 experiment

Preliminary electron microscopy of coated fuel particles from the AGR-1 experiment was conducted using characterization techniques such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and wavelength dispersive spectroscopy (WDS). Microscopic quantification of fission-product precipitates was performed. Although numerous micro- and nano-sized precipitates observed in the coating layers during initial SEM characterization of the cross-sections, and in subsequent TEM diffraction patterns, were indexed as UPd₂Si ₂, no Ag was conclusively found. Additionally, characterization of these precipitates highlighted the difficulty of measuring low concentrations of Ag in precipitates in the presence of significantly higher concentrations of Pd and U. The overlapping x-ray energies did not allow for determination of trace amounts of Ag or for accurate quantitative analysis of the fission product precipitate composition. AGR1 post irradiation examination electron microscopy to date also identified that precipitates in the SiC layer were located on the grain boundaries. The electron microscopy team followed a multi-directional and phased approach in the identification of fission products in irradiated TRISO fuel. The advanced electron microscopy techniques discussed in this paper, not only demonstrate the usefulness of the equipment (methods) as relevant research tools, but also provide relevant scientific results which increase the knowledge about TRISO fuel particles microstructure and fission products transport. The scanning transmission electron microscopy (STEM), TEM, electron back scattered diffraction (EBSD) and electron energy loss spectroscopy (EELS) results are the first of kind results on irradiated SiC layer of TRISO coated particles. Additionally, the application of transmission-EBSD (t-EBSD) is also recognized as the first application of its kind on any TRISO coated particles. Additionally, the STEM characterization provided the first of kind results of silver containing nano-sized triple-points and grain boundaries. This provides significant knowledge for silver transport mechanistic studies which has been the topic of international research for the past forty years.