Nuclear Science User Facilities 74 STEM/EELS Study of Fission Product Transport in Neutron Irradiated TRISO Fuel Particles Haiming Wen – Idaho State University – Tristructural isotropic (TRISO) coated fuel particles are designed for use as nuclear- fuel particles in high temperature nuclear reactors.They are composed of a uranium oxide (UO2) or uranium oxy carbide (UCO) fuel kernel protected by a series of ceramic coating layers that retain fission products.The ceramic TRISO coatings are a finely tuned fission product containment system, which consist of, in the order of increased distance from the fuel kernel, a porous carbon buffer layer, an inner pyrolytic carbon (IPyC) layer, a SiC layer, and an outer PyC (OPyC) layer.The SiC layer is the primary fission product barrier of the TRISO particle and serves as the principal structural layer as well. Release of certain metallic fission products, e.g., Ag and Pd, through intact TRISO coatings has been evident for decades around the world, as well as in the recent Advanced Gas Reactor (AGR)-1 experiment at Idaho National Laboratory.The release of 110mAg is a potential worker safety concern due to plate out on the cooler metallic parts of the helium pressure boundary.This safety concern high- lights the importance of identifying the metallic fission product transport mechanisms of 110mAg through the TRISO coating layers. Project Description The objectives of this project are to study the distribution, composition, and structure of fission products in neutron irradiated SiC layer of theTRISO particles, using electron microscopy including scanning trans- mission electron microscopy (STEM), energy dispersive x-ray spectroscopy (EDS), electron energy loss spectros- copy (EELS), high‑resolution transmis- sion electron microscopy (HRTEM), and precession electron diffraction (PED), in an effort to enhance the understanding of distribution, composition, structure, and transport mechanisms of fission products. In the past 40 years, numerous studies, including reactor experiments, out- of-pile experiments, and simulations, have been performed to investigate the Understanding of fission products in irradiated TRISO fuel particles will facilitate design and fabrication of TRISO particles with improved performance and expedite the development and deployment of high- temperature gas reactors.