Prediction Of Formation Probability Of Rare Earth Uranates Inside Nuclear Reactor Fuel From The Determined Oxygen Potential Using A Solid Oxide Galvanic Cell

Pr6UO12(s) and Er6UO12(s) were synthesized by the citrate nitrate gel combustion method. The synthesis conditions were optimized by characterising the precursors of these uranates heated in the temperature range from 973 to 1473 K using X-ray diffraction analysis and thermo-gravimetric techniques. The solid-state oxide ion conducting galvanic cells were constructed to determine the standard molar Gibbs energy of formation of Pr6UO12(s) and Er6UO12(s) from the measured e.m.f.s in the temperature range of 904-1237 K. The standard molar enthalpy of formation and standard entropies at 298.15 K for RE6UO12(s) (RE = Pr and Er) were derived from the measured Gibbs energy and required data from the literature, using second and third law analysis. The calculated Delta(f)Hom(Pr6UO12,s,298.15 K) and Delta(f)Hom(Er6UO12,s,298.15 K) were -6669 and -6951 kJ mol(-1), respectively. The standard entropy values of Pr6UO12(s) and Er6UO12(s) were calculated as 561 and 563 J mol(-1) K-1, respectively. A similar trend was observed in Delta(f)Hom(RE6UO12,s,T) and Som(RE6UO12,s,298.15 K) to that of RE2O3 along the rare earth series. The oxygen potential and ternary phase diagrams of the Pr-U-O and Er-U-O systems were calculated using the Gibbs energy data measured in this study.