Microstructure and fission products in the UCO kernel of an AGR-1 TRISO fuel particle after post irradiation safety testing

Abstract Tristructural isotropic (TRISO) coated particle fuel, as a key fuel concept for the high-temperature gas-cooled reactors and a candidate accident-tolerant fuel, has been investigated under the US-DOE Advanced Gas Reactor Fuel Development and Qualification Program. Extensive studies have been conducted to evaluate the fission-product release, diffusion of Ag, Pd, and Cd in the SiC layer and the TRISO coating system, and safety-test performance. However, to date there are limited reported results on the fuel kernels’ response to irradiation with or without post irradiation safety testing. To incrementally fill this knowledge gap, extensive studies using transmission electron microscopy (TEM) and atom probe tomography (APT) were conducted on a TRISO fuel-particle kernel with a 19.74% 235U enrichment, irradiated to 18.63% FIMA and subsequently subjected to safety testing at 1600 °C for 300 h. Microstructural characterizations, elemental analysis, and phase identification were conducted using conventional TEM and scanning TEM imaging, energy-dispersive X-ray spectroscopy, selected-area electron diffraction and APT. The following findings were made: (1) significant reconstructions and phase evolutions occurred in the irradiated and post safety-tested fuel kernel, and its microstructure consists of two primary phases, namely a higher-Z (atomic mass) UC phase and a lower-Z UO phase, (2) no fission-gas bubbles are identified within the fuel kernel, that can be attributed to the high-temperature post safety-testing, (3) fission products Zr, Nb, Mo, Ru, Tc and Rh were found to segregate preferentially into UC phase or to form metallic precipitates while the lanthanide fission products tend to stay in the solution of UO phase, and (4) Pd was detected in the rod-shaped precipitates.