Theoretical investigation of thermodynamic stability and mobility of the oxygen vacancy in ThO2–UO2 solid solutions

Abstract

The thermodynamic stability and the migration energy barriers of oxygen vacancies in ThO₂–UO₂ solid solutions are investigated by density functional theory calculations. In pure ThO₂, the formation energy of the oxygen vacancy is 7.58 eV and 1.46 eV under O rich and O poor conditions, respectively, while its migration energy barrier is 1.97 eV. The addition of UO₂ into ThO₂ significantly decreases the energetics of formation and migration of the oxygen vacancy. Among the range of UO₂–ThO₂ solid solutions studied in this work, UO₂ exhibits the lowest formation energy (5.99 eV and −0.13 eV under O rich and O poor conditions, respectively) and Th_0.25U_0.75O₂ exhibits the lowest migration energy barrier (∼1 eV). Furthermore, by considering chemical potential, the phase diagram of the oxygen vacancy as a function of both temperature and oxygen partial pressure is shown, which could help to gain experimental control over oxygen vacancy concentration.