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

Area of Science:

• Nuclear Engineering
• Materials Science
• Computational Physics

Background:

• Accident tolerant fuels (ATFs) are being developed to replace uranium dioxide (UO2) in light-water reactors (LWRs).
• Understanding fission gas behavior, particularly xenon (Xe), is crucial for predicting ATF performance and burnup characteristics.
• Uranium mononitride (UN) and uranium sesquisilicide (U3Si2) are promising ATF candidates.

Purpose of the Study:

• To investigate the thermodynamic behavior of xenon (Xe) and its interaction with vacancies in UO2, UN, and U3Si2.
• To assess the stability and formation energies of Xe-vacancy complexes in these advanced nuclear fuels.
• To provide insights into the defect behavior influencing the performance of accident tolerant fuels.

Main Methods:

• Density functional theory (DFT) calculations using the GGA+U approach.
• Systematic assessment of Xe-vacancy complexes, including interstitial sites and various vacancy clusters.
• Inclusion of corrected chemical potentials to accurately model thermodynamic properties.

Main Results:

• Xe-vacancy complex formation energies differ significantly across UO2, UN, and U3Si2.
• UO2 tends to form vacancy cluster defects with Xe, while UN and U3Si2 favor mono-atomic vacancies and Xe-vacancy complexes.
• Xe confinement is strong in UO2 and UN, but Xe diffuses to larger free volume sites in U3Si2, indicating high gas storage capacity.

Conclusions:

• The study elucidates the distinct interplay between Xe and vacancies in different uranium-based fuels.
• U3Si2 demonstrates superior fission gas storage capabilities, suggesting potential for enhanced fuel performance and safety.
• Findings provide critical thermodynamic data for the design and optimization of accident tolerant fuels.