Prediction of formation probability of rare earth uranates inside nuclear reactor fuel from the determined oxygen potential using a solid oxide galvanic cell

Abstract

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 Δ_fH^o_m(Pr₆UO₁₂,s,298.15 K) and Δ_fH^o_m(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 Δ_fH^o_m(RE₆UO₁₂,s,T) and S^o_m(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.