Stoichiometric Effects on Performance of High-Temperature Gas-Cooled Reactor Fuels from the U-C-O System

Two fuel failure mechanisms were identified for coated particle fuels that are directly related to fuel kernel stoichiometry. These mechanisms are thermal migration of the kernel through the coating layers and chemical interaction between rare-earth fission products and the silicon carbide (SiC) layer (the primary barrier to diffusion of metal fission products out of the particle) leading to failure of the SiC layer. Thermal migration appears to be most severe for oxide fuels, while chemical interaction is most severe with carbide systems. Thermodynamic calculations indicated that oxide-carbide fuel kernels may permit a stoichiometry that reduces both problems to manageable levels for currently planned high-temperature gas-cooled reactors. Such stoichiometry adjustment is possible over the complete spectrum from UO₂ to UO₂ for the present recycle fuel, a weak acid resin (WAR)-derived fissile kernel. Thermodynamic calculations indicate that WAR kernels containing <15% UC₂ (>85% UO₂) will develop excessive CO overpressures within the particle during irradiation. In 100% UO₂ particles, thermal migration and oxidation of the SiC layer were observed after irradiation. The calculations also indicate that WAR kernels containing >70% UC₂ (<30% UC₂) contain insufficient oxygen to oxidize the rare-earth fission products formed in fuel operated to the maximum burnup levels of 75% fissions per initial metal atom (75% FIMA). Instead, the rare earths are present in part or completely as dicarbides. As such, they were observed to segregate from the kernel and collect at the SiC interface on the cold side of the particle, react with the SiC, and eventually fail this coating. Five WAR kernel stoichiometries were irradiated. These are either UO₂ kernels, UO₂ plus 15, 50, or 75% UC₂, or 100% UC₂. Results of these tests are consistent with thermodynamic calculations. Additional tests are in progress to establish the optimum stoichiometry; preliminary indications suggest an optimum value of ∼35% conversion with a permissible range of ±20%.