An improved method to evaluate the “Joint Oxyde-Gaine” formation in (U,Pu)O2 irradiated fuels using the GERMINAL V2 code coupled to Calphad thermodynamic computations

In this work, two different thermodynamic softwares, ANGE using the TBASE database, and OPENCALPHAD using the TAF-ID (Thermodynamics of Advanced Fuels – International Database), have been integrated into the GERMINAL V2 fuel performance code (of the PLEIADES platform) in order to evaluate the chemical state of (U, Pu)O2 fuel and fission products in sodium cooled fast reactors. A model to calculate the composition and the thickness of the “Joint-Oxyde Gaine” (JOG) fission product layer in the fuel-clad gap has been developed. Five fuel pins with a final burnup ranging between 3.8 and 13.4% FIMA (Fissions per Initial Metal Atom) have been simulated, and the calculated width of the fission product layer have been compared with post irradiation examinations. The two different thermodynamic softwares have been compared in terms of computation time and predicted fuel-to-clad gap chemistry. The main elements and phases encountered in the fission productlayer have been identified, and the impact of the changing oxygen potential has been explored.