Modelling nuclear fuel behaviour with TAF-ID: Calculations on the VERDON-1 experiment, representative of a nuclear severe accident

Highlights: • Fission products Ba, Cs, I, Mo, and the UO₂ fuel – cladding interaction during a nuclear severe accident have been calculated with TAF-ID, and compared to experimental results. • Ba and Mo behavior were accurately reproduced, allowing to identify the chemical species that might have been released from the fuel. • TAF-ID calculations accurately predicted the fuel – cladding interactions, and the melting of the resulting solid solution by the end of the experiment. - Abstract: The chemical behaviour of the main elements present in a PWR irradiated fuel sample (UO₂ fuel, Zr from the Zircaloy-cladding, Pu, Np, and fission products such as Ba, Ce, Cs, Gd, I, La, Mo, Nd, Pd, Rh, Ru, Sr, Tc, Te, etc.) submitted to a nuclear severe accident type sequence (VERDON-1 experiment) has been investigated by thermodynamic calculations using the TAF-ID database. Particular emphasis has been placed on the chemical behaviour of the fission products Ba, Cs, Mo, and Zr, and the interactions between the Zircaloy-cladding and the UO₂ matrix. Calculation results have been compared to experimental observations completed during the VERDON-1 experiment (instantaneous release of fission products and final chemical state of elements in the sample). Results presented in this manuscript do not account for non-equilibrium phenomena that have a major impact on fission product behaviour (e.g., chemical diffusion, mass transport induced by temperature gradients in the fuel pellet, etc.). None the less, these TAF-ID calculations have described accurately the melting of the sample following the formation of a mixed (U,Zr)O_2-x phase because of the interaction between the fuel and the Zircaloy-cladding. Furthermore, calculations have assisted in the explanation of the instantaneous released fractions observed for Ba and Mo during the experiment (as a function of the experiment atmosphere and temperature).