First-principles study of Xe-vacancy defect clusters in UC

The formation and binding energetics that control the nucleation behavior of Xe-vacancy defect clusters in uranium monocarbide (UC) is investigated via density functional theory (DFT) calculations. Based on previous work, assumptions on the effective formation energy of U and C vacancies are made, and the substitutional defect XeU is considered as the most stable Xe defect under carbon-rich conditions. The DFT calculations for the binding energy indicate that, depending on the non-equilibrium vacancy concentration under the specific irradiation conditions, clusters can grow by absorbing uranium vacancies, while a saturation effect exists for the absorption of carbon vacancies. It is also found that, depending on the supersaturation and correspondingly, the effective formation energy of XeU under irradiation, the stable nucleus for a Xe bubble consists of a small cluster of 2 or 3 Xe atoms.