Thermodynamic analysis of chemical states of fission products in uranium-zirconium hydride fuel

The chemical state of fission products (FPs) in U+ZrH_1.60 fuel was studied from the thermodynamic point of view. Twenty most abundant FP elements were taken into account in the system of U-Zr-H-O-FP in which oxygen is treated as an impurity. The Thermo-Calc computer code was used to calculate the equilibrium state of the multi-phase and multi-component system. This calculation shows that yttrium, the alkaline earth metals (Ba, Sr) and most of the lanthanides prefer to form corresponding binary hydrides. Oxygen impurities in the system are likely to form a mixture of Y₂O₃, Pr₂O₃, Sm₂O₃ as well as Ce₂O₃ depending on their fission yields. With increasing of burn-up, only a slight decrease of the hydrogen potential in the fuel pin can be expected because of the very little consumption of hydrogen in the hydrogenation process of the FP. In the gas phase, H₂, Cs, Rb, CsRb as well as CsI are the main vapor species apart from the noble gases Xe and Kr. Solid swelling of the fuel due to formation of condensed phases was calculated as a function of burn-up.