Initial examination of fuel compacts and TRISO particles from the US AGR-2 irradiation test

• Published in: Nuclear engineering and design 2018-04, Vol.329, p.89-101
• Main Authors: Hunn, John D., Baldwin, Charles A., Montgomery, Fred C., Gerczak, Tyler J., Morris, Robert N., Helmreich, Grant W., Demkowicz, Paul A., Harp, Jason M., Stempien, John D.
• Format: Article
• Language: English
• Subjects: Actinides; Cesium; Cesium release; Coated particles; Coating; Coatings; Compacts; Data processing; Electron microscopes; Electron microscopy; Fission products; Fuels; High-temperature gas-cooled reactor (HTGR) fuel; Irradiation; Microstructure; Nitric acid; Nuclear fuels; Post-irradiation; Post-irradiation examination (PIE); Radiation; Radiation effects; Retention; SiC failure; Silicon carbide; Structural damage; Tristructural isotropic (TRISO) particles; Uranium; X ray imagery
• ISSN: 0029-5493; 1872-759X
• DOI: 10.1016/j.nucengdes.2017.09.017

•Compacts from the US AGR-2 irradiation test suspected of Cs release were examined.•Acid leaching and particle gamma scanning were used to investigate releases.•TRISO particles with SiC failure were isolated by gamma screening.•SiC failure was studied by X-ray tomography and SEM with elemental analysis.•Microstructure of typical irradiated particles was examined with optical microscopy. Post-irradiation examination was completed on two as-irradiated compacts from the US Advanced Gas Reactor Fuel Development and Qualification Program’s second irradiation test. These compacts were selected for examination because there were indications that they may have contained particles that released cesium through a failed or defective SiC layer. The coated particles were recovered from these compacts by electrolytic deconsolidation of the surrounding graphitic matrix in nitric acid. The leach-burn-leach (LBL) process was used to dissolve and analyze exposed metallic elements (actinides and fission products), and each particle was individually surveyed for relative cesium retention with the Irradiated Microsphere Gamma Analyzer (IMGA). Data from IMGA and LBL examinations provided information on fission product release during irradiation and whether any specific particles had below-average retention that could be related to coating layer defects or radiation-induced degradation. A few selected normal-retention particles and six with abnormally-low cesium inventory were analyzed using X-ray tomography to produce three-dimensional images of the internal coating structure. Four of the low-cesium particles had obviously damaged or degraded SiC, and X-ray imaging was able to guide subsequent grinding and polishing to expose the regions of interest for analysis by optical and electron microscopy. Additional particles from each compact were also sectioned and examined to study the overall radiation-induced microstructural changes in the kernel and coating layers.