THERMOCHEMISTRY OF THE U-O AND ZR-O SYSTEMS

The thermodynamic constraint relating the standard free energy of form ation of an oxide (MO(C)) to its oxygen potentials by DELTAG(f)0(MO(C) ) = 1/2 integral-C/0 DELTAG(O2)BAR dC' has been used to develop the fu ll spectrum of oxygen potentials of the U-O and Zr-O systems at high t emperatures. Henry's law is applied to the metal region in the U-O sys tem. Oxygen dissolution in liquid Zr has been shown to deviate only sl ightly from Henry's law, despite the high solubility. Minor modificati ons of the previously reported oxygen pressures in the two-phase regio ns of both of the U-O and Zr-O systems result from the requirement of satisfying the integral constraint. Oxygen pressures in the oxygen-con taining metal regions where no data are available have also been estim ated. The ensemble of the results is displayed in the form of oxygen i sobars superimposed on the phase diagrams of the U-O and Zr-O systems. The oxygen pressure-temperature-composition relation of tetragonal (b eta) ZrO2+/-x has also been established.