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2014 ANNUAL REPORT 83 factAg was first identified in the SiC layer of a neutron-irradiatedTRISO fuel particle only in 2013 by a Boise State University research team at INL using scanning transmission electron microscopy STEM 1. Project Description As a continuation of the 2013 preliminary STEM study this project utilized STEM and the local elec- trode atom probe LEAP to further investigate the morphology composi- tion and distribution of FPs in the SiC layer of the same neutron-irradiated TRISO fuel particles.The goal was to gain further insight into the FPs transport mechanism particularly that of Ag.The various fission prod- ucts found have improved our under- standing of FP release mechanisms in this type of fuel. Accomplishments STEM-EDS AnalysisTwo transmission electron microscopy TEM lamellae lifted from the SiC layer close to the SiCinnerPyC interface were examined in detail using STEM and energy dispersive spectroscopy EDS.The excellent Z contrast observed under STEM clearly revealed the existence of various types of FP precipitates Figure 1 and EDS aided in the determination of the composition of these precipitates.This study confirmed previous reports of Ag-enriched grain boundary and triple-junction precipitates and identified for the first time the Ag-palladium Pd intragranular precipitates as well as a network of small uranium-rich precipitates.These findings advance our understanding of FP transport through the SiC layer in TRISO fuel particles. LEAP Analysis A batch of 15 LEAP tips from the regions where theTEM lamellae were fabricated and exam- ined in 2013 was prepared.The first phase of this work resulted in some fracturing of the tips primarily due to the weakness of theTEM sample grid on which the samples were placed. The largest data set obtained was only from depths of about 30 m. In the second phase of this study another batch of five tips was fabri- cated and tested in 2014.These tips were made from material taken from the same location as in the first phase but this time they were mounted in a much sturdier copper focused ion beam FIB grid.With the optimized equipment parameters laser energy of 80 to 100 petajoules pJ laser pulse of 160 to 200 kilohertz kHz temperature of 3060 K better grid data could be obtained from deeper in the material than was possible in the first phase of the project.The largest data set obtained is shown in Figure 2. Although the results were superior to those obtained from Phase one of this study the analysis depth was The cutting-edge analytical equipment in the Microscopy and Characterization Suite at CAES can shed new light on thermodynamically and kineti- cally driven nanoscale physical and compositional changes in materials. Dr.Bin LengResearch AssociateUniversity ofWisconsin - Madison