Developing a thermodynamic model for the U-Pd-Rh-Ru quaternary system for use in the modelling of nuclear fuel

Abstract

Ruthenium, rhodium, and palladium are fission products in nuclear fuels. These elements and their compounds change the properties of fuel pellets. Phase diagrams involving uranium have been constructed experimentally to study fission product behaviour, specifically diagrams containing the very stable UMe3 (where Me = Ru, Rh, or Pd) compounds. Discrepancies such as enthalpies of formation of UxMey compounds exist in both experimental binary phase diagram constructions and thermodynamic property determinations. To model the behaviour of fission products in irradiated nuclear fuels, codes (e.g., BISON or RNFTT (RMC Nuclear Fuel Thermochemical Treatment) have been developed. For quantitative studies, existing experimental data are insufficient for such tasks because of difficulties determining data in ranges of composition and temperature. Experimental binary phase diagrams provide phase equilibrium information, yet if not thermodynamically evaluated, the data will be limited in application. Because industrial processes usually involve multicomponent systems that vary in wide ranges of composition and temperature. For some elements with potential catalytic functions (e.g., Pd), the U-Pd phase diagrams presented in the literature were inconsistent so it was a challenge to choose which experimental data should be used for a thermodynamic evaluation. Post irradiation examinations showed that the composition of irradiated nuclear fuels are complicated. For such complex systems, experimental determination of a full set of data is practically impossible. Nevertheless, the possibility of constructing such complex systems by means of thermodynamic evaluation exists. In this work, thermodynamic evaluations of the URu, U-Rh, and U-Pd binary phase diagrams were assessed or re-assessed (e.g., the U-Ru system). In combination with three binary systems previously assessed, a self-consistent quaternary system (U-Pd-Rh-Ru) was constructed. An alternative strategy in optimizing the Gibbs energy functions of various phases, capable of identifying experimental fallacies in hand drawn U-Rh and U-Pd phase diagrams, was proposed. With this quaternary model, two existing ternary experimental phase diagrams were critically evaluated. Results show that without thermodynamic evaluations some experimental data were wrongly interpreted. The establishment of the quaternary model enriches thermodynamic databases and will potentially improve the performance of the RNFTT treatment and codes such as BISON.