Free energy of Xe incorporation at point defects and in nanovoids and bubbles in UO2

Intra- and intergranular fission gas bubbles in nuclear fuels are known to have a deleterious effect on fuel performance, particularly at high levels of burnup. The mechanisms by which randomly distributed fission gas atoms agglomerate to form larger fission bubbles are not well understood. Therefore, this paper aims to examine the thermodynamics of bubble nucleation from isolated point defects to nanovoids and ultimately to bubbles of approximate to 2.0 nm using molecular-dynamics simulations employing empirical pair potentials. A thermodynamic driving force for bubble nucleation from point defects is highlighted by the substantial reduction in the free energy of Xe atoms contained within larger bubbles relative to accommodation at point defects. The simulations also illustrate the processes that the lattice surrounding a fission gas bubble undergoes in order to prevent thermal resolution, clearly indicating the thermodynamic imperative to ensure the Xe remains in the bubble.