Molecular dynamics study of Xe bubble re-solution in UO ₂

In-pile diffusion of fission gas is generally modeled through an effective diffusion coefficient that must take into account various phenomena occurring at the grain scale. One of these relates to the trapping of gas by intragranular bubbles. This trapping is only temporary, because the gas is brought back into atomic solution through interactions with fission fragments or fast neutrons, the so-called resolution process. In this work, we investigate the resolution process with molecular dynamics techniques. For low-energy interactions (<50 keV), the collision cascades caused by a single fast moving primary knock-on atom (PKA) were studied. The higher energy range (up to 100 MeV total energy, but limited to an effective linear deposited energy between 10 and 40 keV/nm) was studied by the thermal spike method. The studies shed light on the atomic scale processes at play for fission gas resolution in nuclear fuel. The present atomic scale simulations indicate that low-energy interactions (PKA) are not effective in the resolution process. The high energy interactions destroy smaller bubbles completely and bring a quasi-constant number of gas atoms in resolution when they interact with larger bubbles.