Interaction between Xe bubbles and grain boundaries as well as the influences on structural evolution in UO₂: A molecular dynamics simulation

In polycrystalline UO₂ fuels, fission Xe bubbles not only cause fuel swelling, but also are released into the fuel-cladding gap through GBs. The accumulation of a large amount of Xe might reduce the thermal conductivity of the gap and increase the pressure of the fuel rods, leading to fuel failure and cladding cracking. In order to further optimize the design of nuclear fuels, it is crucial to understand the effects of GBs on the thermophysical properties and dynamic behaviors of Xe bubbles. In this work, molecular dynamics simulations were performed to study the interaction between Xe bubbles and STGBs, as well as the influences on the structural evolution of Xe bubbles and GBs in UO₂. It was found that the structure order and pressure in Xe bubbles increase with the Xe/(V_U+V_O) (U and O vacancies) ratio up to 1.0, but decrease beyond that Xe/(V_U+V_O) ratio. The larger the Xe bubbles are, the more disordered is the atomic structure, and the lower is the pressure in Xe bubbles. However, the atomic structure in Xe bubbles becomes more disordered and the pressure in Xe bubbles becomes lower when they are close to GBs in UO₂, which is not conducive to the heat transfer inside UO₂ fuels and lowers the utilization efficiency of fuels. Generally, the migration of high-angle STGBs is slow, but will be accelerated when Xe atoms are gradually diffused into GBs and migrate with GBs together. By comparison, the low-angle STGBs in UO₂ could migrate easily, but will be slowed down when the dislocations in GBs are pinned by Xe bubbles.