A first-principles investigation of internal energy and entropy of formation of charged defects in Th1−xUxO2 (x ≤ 0.5)

Abstract

Mixed thorium/uranium dioxide, (Th,U)O2, is under consideration for advanced nuclear fuel applications. Investigating the point defect structure and energy in this oxide is important for predicting its behavior as fuel. In this work, we use first-principles calculations based on the generalized gradient approximation (GGA)+Hubbard U approach to investigate the internal energy and entropy of the formation of point defects in Th1−xUxO2 at various compositions below x ≤ 0.5. Point defects including O vacancies, O interstitials, Th vacancies, Th interstitials, U vacancies, and U interstitials have all been considered with their charges ranging from neutral to the maximum nominal values. The observed trends have been explained in terms of electronic density of states. The valence band maxima of crystals that contain defects play a crucial role and exhibit variations depending on the U content and the applied charge. The temperature dependence of internal energy and entropy of formation of defects have also been examined. The internal energy of formation of defects was found to exhibit slowly varying or constant values with respect to changes in the U content, except at low values of x and low temperatures. The entropy of formation of defects was observed to decrease with increasing U content. It was additionally observed that the entropy of formation of vacancies increases with temperature, while that of interstitials decreases. This investigation further revealed that at 0 K, the cation vacancies and anion interstitials become increasingly favorable with increasing U content, while cation interstitials and anion vacancies become less favorable.