Kernel migration in failed, oxide-based HTGR fuel

Oxide-based HTGR fuel particles with artificially failed coatings have been heated in the range 11000 to 13500C (1273 to 1623 K) to investigate the interaction of the kernel with its surrounding coating. At these temperatures, oxide-based kernels in failed TRISO-coated particles migrate due to the reaction of the oxide kernel with the surrounding carbonaceous layers. The rate of kernel migration is dependent on the time of reaction, the temperature, and the CO pressure. The temperature dependence of the migration rate suggests that migration is controlled by diffusion of oxygen across the carbide product layer. Adding CO to the cover gas reduces the rate of migration and can prohibit migration if the CO pressure exceeds the thermodynamic equilibrium value for a given reaction temperature. Because complete conversion of the oxide kernel to carbide occurs within the particle, migrating oxide-based kernels do not penetrate the silicon carbon (SiC) coating in TRISO particles. Thus, propagating particle failure caused by kernels that have breached their coatings reacting with the outer coatings of adjacent particles will not occur in the reference particle at normal high temperature gas-cooled reactor (HTGR) operating temperatures