PIE on Five Irradiated AGR-1 Compacts

Abstract

Post-irradiation examination (PIE) is in progress on coated particle fuel compacts from the first Advanced Gas Reactor irradiation experiment (AGR-1). PIE has been completed on five compacts at the Oak Ridge National Laboratory (ORNL). Three compacts (Compacts 6-1-1, 4-4-2, and 5-2-3) were examined in the as-irradiated condition and two compacts (Compacts 6-4-3 and 3-3-2) were subjected to safety testing at 1600°C, followed by post-safety testing examination similar to the PIE performed on the as-irradiated compacts. All compacts were subjected to a standard set of analyses that included the following: (1) detection of exposed fission products by Deconsolidation-Leach-Burn-Leach (DLBL), (2) measurement of gamma-emitting fission product inventory using the Irradiated Microsphere Gamma Analyzer (IMGA), and (3) microstructural examination by x-ray tomography and materialography. The equipment and methods used are reported in detail in ORNL/TM-2012/233, AGR-1 Irradiated Compact 6-1-1 PIE Report, and have been summarized in the next section of this report. This report also provides a brief summary of the results of the completed PIE. Preliminary trends are discussed in this report, but final conclusions will require the support of additional data and further analysis. A separate report is being generated for each AGR-1 compact after PIE is complete that describes in detail the examinations performed and fully documents the data obtained. A final AGR-1 PIE summary report will compile all available data and provide a more comprehensive analysis of the AGR-1 fuel performance. In general, it was observed that the tristructural isotropic (TRISO) coated particles remained intact during irradiation and subsequent safety testing to 1600°C. No significant gaseous 85Kr release was detected during the irradiation or high temperature safety testing, which would be indicative of simultaneous failure of all three outer layers of the TRISO coating. Only three particles with defective SiC layers were detected in the five compacts included in this summary. These particles were detected because they released a significant fraction of their cesium inventory, which was generally well retained. Two particles from Compact 5-2-3 released cesium during irradiation and one particle from Compact 3-3-2 released cesium during safety testing. However, it is evident that some fission products were released through intact SiC during irradiation. Most of these fission products were trapped in the compact matrix, where they could be analyzed by PIE. In terms of fractional release, the main fission products detected outside intact SiC were silver, palladium, and europium. The main impact of irradiation on the microstructure of the TRISO-coating layers was densification and shrinkage of the buffer layer, which often led to buffer fracture and subsequent enhanced swelling of the kernel. Some inner pyrocarbon (IPyC) layers were cracked by the radiation-induced changes in the buffer layer, and on rare occasion these cracks propagated into the SiC. In the two particles that released cesium during irradiation, these SiC cracks penetrated through the entire layer. The particle from Compact 3-3-2 that released cesium was not directly related to irradiation damage. Its SiC layer was determined to be porous as a result of a fabrication defect that occurs when a particle is over-fluidized in the coater, causing it to pick up carbon soot on the surface of the IPyC prior to SiC deposition. This porous SiC layer remained sufficiently intact during irradiation to retain cesium, but finally cracked during high temperature testing. Analysis of fission products remaining in the coated particles showed that the retention of silver varied dramatically from particle-to-particle within each compact. Examination with the IMGA determined that some particles contained a level of 110mAg equivalent to that calculated to have been generated over the three-year irradiation period, while other particles contained too little to be detected by the gamma analysis. The mechanism for silver release is still being investigated. Attempts to correlate the varied release behavior to obvious radiation-induced changes in the coating layers have not been successful. Scanning electron microscopy (SEM) has provided some evidence that particles with higher silver release also show greater penetration of palladium silicide into the SiC layer, and this should be studied further.