Modeling high burnup fuel thermochemistry, fission product release and fuel melting during the VERDON 1 and RT6 tests

This paper presents simulations of the VERDON 1 and RT6 tests (temperature increase up to fuel-clad melting, oxidizing and/or reducing conditions within the furnace) performed with high burnup UO2 fuel (i.e., up to 72 GWd/tU) and considering a coupling between irradiated fuel thermochemistry and a fission gas release model. The thermochemical calculations rely on the Thermodynamics of Advanced Fuels - International Database (TAF-ID) for the description of the phases likely to form from the 15 fission product considered in the fuel (Ba, Ce, Cs, I, La, Mo, Nd, Np, Pd, Rh, Ru, Sr, Tc, Te, Zr) and on the OpenCalphad solver for the minimization of the Gibbs energy of the system. The gas release model describes a diffusion process for the gases within equivalent spherical grains. It accounts for the gases formed by reaction between the fission products as well as the chemically inert noble gases. The progressive fission product depletion of the fuel due to their release in gas form closes the coupling. The impact of the fission product release kinetics on the thermochemical equilibria within the fuel is then studied by coupling the thermochemical calculations with a gas diffusion model. The coupled simulations led to a very good agreement with the release kinetics of various fission products (I, Te, Cs, Mo, Ba). The released fractions of the low-/non-volatile fission products and of uranium, plutonium are also well reproduced. The observed differences in the fuel-clad melting temperatures in the two tests (2200 °C during RT6 and 2600 °C during VERDON 1) are however not reproduced by the simulations.