Effect of ultra-high temperature treatment on microstructures and mechanical properties of TRISO particles

Tristructural-isotropic (TRISO) fuel particles have long been widely used in high temperature gas-cooled reactors (HTGR), where their high temperature mechanical integrity and ability to retain fission products even in the accident conditions are the critical safety issues. In the present work, TRISO particles were subjected to ultra-high temperature thermal treatments at 1900, 2000, and 2200 °C, respectively, which were much higher than the projected accident temperature (1620 °C). Phase compositions and microstructures of the samples before and after thermal treatments were studied by X-ray diffraction (XRD), Raman spectroscopy, and Scanning Electron Microscopy (SEM). Nano hardness and Young`s modulus of the SiC layer in TRISO particles were studied by nano indentation technique, and crushing strength of the TRISO particles was evaluated by mechanical crush tests. The results show that, there were no obvious temperature effects on phase composition and microstructure, except for SiC layer in the sample thermal treated at 2200 °C, in which many pores caused by SiC decomposition were observed. The nano hardness and Young`s modulus values of SiC layer in the as-received and thermally treated TRISO particles were similar within the experimental error. On the other hand, the crushing strength of the thermally treated TRISO particles decreased greatly as compared with the as-received particles, but the crack propagation mode changed from through-the-particle to deflection inside the buffer PyC layer or between buffer PyC and inner PyC (IPyC) layers. The change of crack propagation mode could probably due to the shrinkage of PyC layers due to anisotropy at high temperature and ordering of graphitization of IPyC layers, which probably weakened the interfacial bonding between the IPyC and the buffer PyC layers. The results of this study provide a database reference for the performance of TRISO particles under ultra-high temperature operation and even accident conditions.