Thermochemical bounds on UCO TRISO kernel compositions

Highly-detailed equilibrium thermodynamics calculations were used to derive bounds on safe fresh UCO TRISO fuel kernel chemistries, in terms of the fraction of UC_x. The lower bound on UC_x was chosen to limit CO pressure, and the upper bound to ensure oxidation of radionuclide fission and transmutation products of concern. The bounds can be calculated with respect to both target burnup and fuel temperature. Calculated bounds for a demonstration fuel design similar to the AGR-2 experiment are in good agreement with previously-calculated lower bounds and experimental evidence used to determine upper bounds, and indicate that reference fuel designs such as AGR-2 and MHTGR-350 are well-within the safe composition region even for burnup as high as 21.3% FIMA. Predicted speciations of cesium, palladium, silver, barium, strontium, and zirconium were examined, and recommendations for thermodynamic assessments of relevant systems provided.