Understanding xenon and vacancy behavior in UO2, UN and U3Si2: a comparative DFT+U study

Abstract

Extensive attention has been paid to accident tolerant fuels (ATFs), such as uranium mononitride (UN) and uranium sesquisilicide (U₃Si₂), which are regarded as potential candidates to replace uranium dioxide (UO₂) in light-water reactors (LWRs). However, the thermodynamic behavior of fission gas atoms in these fuels that can quantitatively affect the burnup characteristics of ATFs needs to be explored. To this end, systematic density functional calculations on the energetic properties of xenon (Xe)-vacancy complexes in UO₂, UN and U₃Si₂ are performed with the GGA+U approach as well as the corrected chemical potential. The stabilities of Xe-vacancy clusters, including interstitial trap site (IS), mono-, bi- and tri-atomic vacancies, are thoroughly assessed. The formation energies of vacancy complexes indicate that they are more likely to form vacancy cluster defects and their complexes with Xe in UO₂ and to generate mono-atomic vacancy and Xe-vacancy complexes in both UN and U₃Si₂. Xe can be strictly confined by the trap sites in UO₂ and UN, and yet in U₃Si₂, it prefers to move to the centre of a large free volume trap site. The strong solubility of Xe in U₃Si₂ indicates the excellent storage capacity of fission gas products in the matrix. Overall, this work provides comprehensive insights into the origins of the interplay between Xe and vacancies as well as the thermodynamic behavior of defects in uranium-based fuels.