2016 | ANNUAL REPORT 77 The advanced electron microscopy techniques available at CAES provide great tools for studying structure and composition of fission products in neutron irradiated nuclear fuels. — HaimingWen, Research Assistant Professor fission product transport behavior and mechanisms. Grain boundary (GB) diffusion, neutron induced diffu- sion, catalytic Pd assisted transport, and vapor-phase migration all have been suggested as possible governing transport mechanisms.The previous studies, however, have not been able to reproduce the transport behavior of fission products and the mechanisms remain poorly understood.This project is anticipated to contribute to an enhanced understanding of the mecha- nisms of fission product transport in the irradiated SiC layer ofTRISO parti- cles.Thorough understanding of fission product transport mechanisms in the SiC layer will improve fuel modeling and design, and better SiC layers and TRISO coated particles can be designed to enhance retention of fission prod- ucts and reduce their release. Eventually, TRISO coated particles with improved performance can be fabricated and this advancement will expedite the quali- fying and licensing processes forTRISO coated particles. Hence, this research is contributing to achieving the objectives of U.S. Department of Energy Office of Nuclear Energy (DOE-NE) to develop the next generation nuclear reactors, particularly high temperature gas reactors. Accomplishments The technical goals of the research are to determine the distribution, composition, and structure of fission products in the SiC layer of a TRISO particle, neutron-irradiated to 3.6 × 1021 n/cm2 fast fluence at 1040°C, utilizing electron microscopy methods. STEM was used to investigate distribution of fission products; EDS in STEM was employed to obtain fission product composition; information on structure of fission products was acquired using HRTEM; PED derived ASTAR was executed to study characters of grain boundaries where fission products are located; attempts were made to measure charge states of precipitates using EELS.The experiments and data analyses were conducted by Haiming Wen, and the samples were from the Advanced Microscopy and Microanalysis Program that Isabella van Rooyen is in charge of under the Advanced Gas Reactor Program of DOE-NE. Both Wen and van Rooyen participated in the design of the project. Accomplishments were made toward the technical goals. In summary, a high density of nanoscale fission product precipitates was observed in the SiC layer close to the SiC-IPyC interface, most of which are rich in Pd, while Ag was not identified. Some Pd‑rich precipitates contain U. Precipitates tend to have complex structure.Although a precipitate appears to have uniform contrast in STEM, it may exhibit non-uniform contrast inTEM/HRTEM, which may be caused by composition variations