Simulation of the chemical state of high burnup (U,Pu)O2 fuel in fast reactors based on thermodynamic calculations

In this paper, the chemical state of fast reactor (U,Pu)O2 fuel at high burnup has been simulated using OpenCalphad software and the TAF-ID thermodynamic database. This has been done in order to evaluate the combination of software and database for further implementation into the Germinal fuel performance code of the Pleiades simulation platform. The results have been compared with post irradiation examinations (PIE) of fuel samples from the Phénix sodium-cooled fast reactor. The calculations performed from isotopic data compositions were able to predict all precipitates encountered in the PIE, as well as several other phases. When possible, the measured composition of the phases were compared with the simulations, and show a good similarity in this regard. Additionally, calculations based on measured composition in the fluorite (U,Pu)O2 phase have been performed at different temperatures and oxygen-to-metal ratios. Here, the calculations predict that the formation of fission product oxide compounds occurs to a greater extent in the cooler fuel periphery, which is also what experiments have shown. Oxygen potential has been calculated and compared with experiments with similar composition. The calculations are considered fast and reliable enough for implementation of the thermodynamic software into the fuel performance code.