Migration of ThO₂ Kernels Under the Influence of a Temperature Gradient

Biso-coated ThO₂ fertile fuel kernels will migrate up the thermal gradients imposed across coated particles during high-temperature gas-cooled reactor (HTGR) operation. Thorium dioxide kernel migration has been studied as a function of temperature (1290 to 1705°C) (1563 to 1978 K) and ThO₂ kernel burnup (0.9 to 5.8% FIMA) in out-of-pile postirradiation thermal gradient heating experiments. The studies were conducted to obtain descriptions of migration rates that will be used in core design studies to evaluate the impact of ThO₂ migration on fertile fuel performance in an operating HTGR and to define characteristics needed by any comprehensive model describing ThO₂ kernel migration. The kinetics data generated in these postirradiation studies are consistent with in-pile data collected by investigators at Oak Ridge National Laboratory, which supports use of the more precise postirradiation heating results in HTGR core design studies. Observations of intergranular carbon deposits on the cool side of migrating kernels support the assumption that the kinetics of kernel migration are controlled by solid-state diffusion within irradiated ThO₂ kernels. The migration is characterized by a period of no migration (incubation period), followed by migration at the equilibrium rate for ThO₂. The incubation period decreases with increasing temperature and kernel burnup. The improved understanding of the kinetics of ThO₂ kernel migration provided by this work will contribute to an optimization of HTGR core design and an increased confidence in fuel performance predictions.