THERMODYNAMICS OF THE ZR-H SYSTEM

The consistencies among the reported univariant hydrogen pressures in the three two-phase regions of the Zr-H system, alpha Zr/delta ZrH2-x, alpha Zr/beta Zr, and beta Zr/delta ZrH2-x, are evaluated with the th ermodynamic constraint derived by Speiser.(16) Hydrogen dissolution in alpha Zr is evaluated by checking the consistencies among the Sievert s' law constant, the terminal solubility, and the corresponding two-ph ase univariant pressures involving this phase, A modified Sieverts' la w is proposed for the complicated behavior of hydrogen dissolution in beta Zr. An analytical composition-temperature-pressure relationship f or delta ZrH2-x is established by using the quasi-chemical model deriv ed by Arita et al. The thermodynamic constraint of the free energy of formation of ZrH2 shows that the value measured by Fredricken et al, i s the most reliable, In addition, a more accurate free energy of forma tion of zirconium hydride at the upper phase boundary of alpha Zr/delt a ZrH2-x two-phase region is suggested.