Fission Products Chemistry in Simulated PWR fuel up to 2100 °C: Experimental Characterisation and TAF-ID Modelling
Résumé
The capabilities of the Thermodynamic of Advanced Fuels - International Database (TAF-ID) for reproducing the chemical behaviour of irradiated nuclear fuel in severe accident conditions are studied in this work, by comparing calculation results to experimental observations. To this end, Simfuel samples that are representative of Pressurized Water Reactors (PWR) fuel irradiated up to 76 GWd•tU-1 were submitted to 1327 °C in oxidizing conditions, and to 1800, 2000, and 2100 °C under reducing ones. Characterization of the UO2 fuel and Fission Products (FP) surrogates (Ba, Ce, La, Mo, Nd, Pd, Rh, Ru, Sr, and Zr) was done by Electron Probe Micro Analysis (EPMA), High-Resolution X-Ray Diffraction (HR-XRD), and X-Ray Absorption Spectroscopy (XAS). Experimental characterization confirmed (i) the solubility of La, Y, Nd, Ce and to some extend of Sr and Zr in the UO2 fluorite matrix; (ii) the presence of a perovskite phase, (Ba,Sr)(Ce,Mo,Zr,U)O3, in all the samples but which composition is highly dependent on experimental conditions; (iii) the presence of metallic U in reducing conditions, in association with Ru, Rh, and Pd; and (iv) metallic Mo precipitates, in association with Ru and Pd. In addition, a (U,Zr)O2 phase with a non-homogeneous distribution of U and Zr was observed in reducing conditions. Thermodynamic calculations not only reproduce very well the observed phases and the evolution of their composition with temperature and atmosphere, but also allow explaining their microstructure as a result of the progressive solidification of liquids. In addition, calculations explain the non-homogenous distribution of elements in the (U,Zr)O2 phase, which is attributed to the separation of a homogeneous fluorite phase at high temperature into fluorite and monoclinic-ZrO2 phases at low temperature. However, some features such as the U-Pd-Rh association could not be reproduced by calculations, due to the lack of a thermodynamic model for this ternary system. Altogether, these results demonstrate the great potential of the TAF-ID for the prediction of irradiated fuel behaviour in normal and off-normal reactor operating conditions.