The role of dopant charge state on defect chemistry and grain growth of doped UO2

Abstract

Additives are widely used to control the microstructure of materials via their effect on defect chemistry during sintering. As the primary nuclear fuel, the properties of UO2 are crucial for safe and efficient reactor operation. UO2 has been manipulated by fuel vendors through doping to enhance grain size to provide improved fission gas retention and plasticity. In this work the common phenomenon that governs the effect of Mg, Ti, V, Cr, Mn, and Fe doping of UO2 for enhanced grain growth is revealed, elucidating experimental observations. A combined density functional theory and empirical potential description of defect free energy is used to calculate the doped UO2 defect concentrations as a function of temperature. At high (sintering) temperatures all dopants studied transition to a positively charged interstitial defect. Furthermore, a number of dopants (Ti, V, Cr, and Mn) do so in sufficiently high concentrations to greatly increase the negatively charged uranium vacancy concentration. High uranium vacancy concentrations can enhance grain growth and fission gas diffusion. Mg and Fe also enhance uranium vacancy concentrations but to a lesser extent, while Al has no impact. The enhanced uranium vacancy concentrations, associated with solution of dopants interstitially, is proposed as the mechanism responsible for the enlarged grains seen experimentally in (Ti/V/Cr/Mg)-doped systems. Mn- and V-doped UO2 have been predicted to have higher uranium vacancy concentrations than the more widely used Cr-doped UO2, leading to higher grain growth and fission gas diffusivity.