Molecular dynamics simulations of uranium dioxide with fission gas and under radiation

Abstract

The results of radiation displacement presented three phases for the evolution of radiation process: (1) Primary Knock-on Atoms (PKA) collisions phase, (2) thermal spike phase, and (3) relax phase. Quantified analysis showed that high energy PKA could generate much more severe damage than lower energy PKA could. It also suggested that PKA is the primary cause of the displaced atoms and interstitial atoms, while fission byproduct Xe is the primary cause for the vacancies. The results of radiation displacement also indicated that the existence of Xe is against the growth of the oxygen interstitial clusters.

The simulation results of single Xe atoms' diffusion suggested that Xe atom is oscillating, instead of migrating, around its equilibrium position. In another hand the results about the nucleation of Xe atoms indicated that Xe atoms tend to gather together in UO2 lattice.

The results of melting process suggested the melting point for pure UO2 to be 3200 K ± 12.5 K, and UO2 with fission gas (corresponds to 0.005 FIMA) to be 3175 K ± 12.5 K, respectively. The results also illustrated that the melting point of UO2 is descending with the increase of Xe's density at a descending rate around 15 K / 0.0025 FIMA.

Uranium Dioxide (UO2) is widely used in nuclear industry as the nuclear fuel. The thermodynamic properties of UO2 are of essential interest and of primary importance for the current safety and accident analysis of nuclear fuel.

The objective of this research is to study the thermodynamic properties of UO2 with the existence of fission byproduct, Xenon (Xe), and under radiation through Molecular Dynamics (MD) simulation. Morelon's, Yakub's, Geng's and ZBL potentials were utilized as the interatomic potentials for the simulations. Lattice parameter, density, enthalpy, thermal conductivity, mean square displacement, oxygen diffusivity, and activation energy were calculated and compared to the experimental results. The radiation displacement process and melting process were investigated with different radiation energies and Xe concentrations.

The results of lattice parameter, density, and enthalpy of UO2 presented good agreement with former experimental data and theoretical predictions. As temperature increased from 273 K to 3000 K, the density of UO2 dropped from 11.0 g/cm3 to 9.7 g/cm3, and the enthalpy of UO2 increased from near-zero to 250 kJ / mol. The calculated thermal conductivities of UO2 based on the Yakub potential gave a good agreement in the middle temperature range with the experimental data. The calculated MSD of uranium and oxygen presented the poor mobility of uranium sub-lattice at all temperatures and the high mobility of oxygen sub-lattice towards 3000 K. By fitting the diffusion coefficient of oxygen with VFT law and ARR law, a secondary phase transition near 2006.4K was observed. By fitting the oxygen diffusion constant with ARR equation, the activation energies of 2.0 eV and 2.7 eV were obtained.