Equation of state for helium-xenon gas mixture studied by molecular dynamics simulations

In the framework of long term behaviour of nuclear spent fuels, the performance codes need physical data base such as fission gas equation of state in order to assess the pressure inside fission gas bubbles. The high pressures expected in such bubbles require gas equation of state, which are not easily accessible in the literature. Therefore, molecular dynamics simulations are carried out to assess the helium-xenon mixture equation of state in these extreme conditions. First, we confirm that the interatomic potentials used, which are key parameter for the molecular dynamics simulations, reproduce very well the existing experimental data of the pure gases. This first study provides also new data for the melting point of pure xenon in the range of 500 K to 1500 K under high pressures. Second, the simulations of the helium-xenon mixture confirm that this mixture behaves like van der Waals compounds with the presence of a gas-gas separation in equilibrium at high densities. The miscibility gap, already known for temperatures under 333 K, was extended at higher temperatures until 800 K for high gas densities. This behaviour yields to the creation of an heterogeneous gas mix, which could curb the creation of bubble containing this mixture. Finally, we have fitted with the Happel formalism a new equation of state for the helium-xenon gas mixture, which is valid over temperatures ranging from 200 K to 1000 K and for atomic volumes higher than 0.27 x 10-28 m3 (i.e., densities lower than 6.1 x 104 mol.m-3).