Thermodynamic stability of ThZr₂H_x at high temperature

ThZr₂H_x is the major component of the U-Th-Zr-H fuel, which is considered as a new nuclear fuel. The heats of formation of ThZr ₂H_x (x=4 or 6) were evaluated using two semi-empirical thermodynamic models, i.e. the Miedema's and Griessen-Driessen's models. Compared the evaluated results with the experimental data, it is shown that the Griessen-Driessen model predicts the heat of formation more precisely than the Miedema model does. By taking the entropy change into consideration, the Gibbs free energy of formation of ThZr₂H_x has been derived. The hydrogen pressures of ThZr₂H_x were evaluated by thermodynamic analysis based on the Gibbs free energy of formation. The calculation results show that the ThZr₂H_x has low hydrogen pressure at high temperature compared with the TRIGA fuel. It is concluded that the ThZr₂H_x has an advantage in setting the limitation of operation power under normal operation condition and is more advantageous in accident condition for assuring the safety operation of nuclear reactor.