Release of radionuclides from intact tristructural-isotropic (TRISO) coated particle fuel at normal and accident conditions is a primary metric of fuel performance. The distribution of fission products and actinides in the TRISO layers of individual particles provides insight on radionuclide transport and release behavior and was determined using scanning electron microscopy analysis. Particles were isolated from an irradiated fuel compact (AGR-1 Compact 4-4-2) and analyzed as-irradiated or after individual particle safety-testing at 1800 °C for 650 h. Particles were selected for comparison based on their remaining ^110mAg fission product inventory. These comparisons corroborated the observation that the ^110mAg inventory is a marker for relative irradiation temperature based on observed radionuclide distribution in the SiC layer. The comparison also indicated that the in-pile behavior influences the fission product and actinide species interactions with the TRISO layers during high temperature exposure after irradiation. The analysis confirms both palladium and uranium diffusion, as well as other species, are active in the UCO TRISO fuel system at 1800 °C and that palladium transport is active at lower temperatures relative to uranium. While diffusion across the SiC layer was observed, the intact nature of the SiC layer after the 1800 °C, 650-h exposure indicates the SiC layer maintained its functionality as a fission product barrier by mitigating release of radionuclides at beyond accident margin temperatures.

在正常和事故情况下，从完整的三向同性（TRISO）涂层颗粒燃料中释放放射性核素是燃料性能的主要指标。裂变产物和act系元素在单个粒子的TRISO层中的分布提供了对放射性核素传输和释放行为的洞察力，并使用扫描电子显微镜分析法进行了测定。从辐照过的燃料块（AGR-1 Compact 4-4-2）中分离出颗粒，并进行辐照或在1800°C下进行了650小时的单独颗粒安全性测试后进行分析。根据其剩余的^110m Ag裂变产物库存选择颗粒进行比较。这些比较证实了^110mAg存量是基于观察到的SiC层中放射性核素分布的相对辐照温度的标记。比较还表明，在辐照后的高温暴露期间，堆中行为会影响裂变产物和act系元素与TRISO层的相互作用。分析证实，钯和铀以及其他物质在UCO TRISO燃料系统中在1800°C时都具有活性，并且钯的运输在相对于铀较低的温度下也很活跃。在观察到整个SiC层扩散的同时，暴露于1800°C，650小时后SiC层的完整性质表明SiC层通过减轻放射性核素在事故边缘温度以外的释放而保持了其作为裂变产物屏障的功能。