Understanding and Predicting the Thermodynamic Behavior of Fission Products Encountered Between the (U,Pu)O2 Fuel Pellet and the Cladding: Characterization and Modeling Approaches

This study provides a comprehensive investigation of the formation and behavior of a layer enriched in fission products encountered between the (U,Pu)O₂ fuel pellet and the cladding, designated as JOG (“Joint Oxyde Gaine” in French). Employing a multifaceted approach that combined thermodynamic calculations, experimental synthesis, and advanced characterization techniques, simulated JOG has been synthetized (without radioactive fission products). Using thermodynamic calculations with the TAF-ID database, the phase compositions in the JOG was assessed for various temperatures, pressures, and oxygen potential conditions, revealing insights into the environmental factors influencing JOG formation. Experimental simulation of the JOG composition, exposed to controlled conditions, confirmed the presence of key compounds such as Cs₂MoO₄, CsI, and PdTe, as evidenced by SEM, EDS, and XRD analyses. The results of the calculations highlighted notable differences in the nature of the phases constituting the JOG under varying pressure and oxygen potential conditions. At 873 K and oxygen partial pressure of 10^–4 bar, Cs₂MoO₄, Pd–Te, and a gas phase rich in tellurium and CsI were predominant, contrasting with the emergence of liquid phases at 70 bar. This study offered a comprehensive understanding of JOG microstructure, and highlighting the importance of accurate characterization for reactor safety. This information lays the foundations for future studies on the chemical interaction between the JOG and the steel cladding.