Thermodynamic stability of ThZr2Hx at high temperature

ThZr2Hx is the major component of the U-Th-Zr-H fuel, which is considered as a new nuclear fuel. The heats of formation of ThZr2Hx (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 ThZr2Hx has been derived. The hydrogen pressures of ThZr2Hx were evaluated by thermodynamic analysis based on the Gibbs free energy of formation. The calculation results show that the ThZr2Hx has low hydrogen pressure at high temperature compared with the TRIGA fuel. It is concluded that the ThZr2Hx 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.