Interplay of defect cluster and the stability of xenon in uranium dioxide by density functional calculations

TL;DR

This study uses first-principles calculations to explore how defect clusters influence xenon behavior in uranium dioxide, revealing thermodynamic preferences and charge states that impact bubble growth in nuclear fuel.

Contribution

It provides new insights into xenon defect interactions in UO2 under realistic conditions, extending previous models with detailed thermodynamic and charge state analysis.

Findings

Oxygen defect clusters increase xenon solution energy by 0.5 eV.

Xenon prefers tri-vacancy sites in hypo-stoichiometric conditions.

Uranium vacancies are favored for xenon in hyper-stoichiometric regimes.

Abstract

Self-defect clusters in bulk matrix might affect the thermodynamic behavior of fission gases in nuclear fuel such as uranium dioxide. With first-principles LSDA+U calculations and taking xenon as a prototype, we find that the influence of oxygen defect clusters on the thermodynamics of gas atoms is prominent, which increases the solution energy of xenon by a magnitude of 0.5 eV, about 43% of the energy difference between the two lowest lying states at 700 K. Calculation also reveals a thermodynamic competition between the uranium vacancy and tri-vacancy sites to incorporate xenon in hyper-stoichiometric regime at high temperatures. The results show that in hypo-stoichiometric regime neutral tri-vacancy sites are the most favored position for diluted xenon gas, whereas in hyper-stoichiometric condition they prefer to uranium vacancies even after taking oxygen self-defect clusters into account at low temperatures, which not only confirms previous studies but also extends the conclusion to more realistic fuel operating conditions. The observation that gas atoms are ionized to a charge state of Xe+ when at a uranium vacancy site due to strong Madelung potential implies that one can control temperature to tune the preferred site of gas atoms and then the bubble growth rate. A solution to the notorious meta-stable states difficulty that frequently encountered in DFT+U applications, namely, the quasi-annealing procedure, is also discussed.