U and Xe transport in UO2±x: Density functional theory calculations

Abstract

The detrimental effects of the fission gas Xe on the performance of oxide nuclear fuels are well known. However, less well known are the mechanisms that govern fission gas evolution. Here, in order to better understand bulk Xe behavior (diffusion mechanisms) in UO2±x we calculate the relevant activation energies using density functional theory (DFT) techniques. By analyzing a combination of Xe solution thermodynamics, migration barriers and the interaction of dissolved Xe atoms with U, we demonstrate that Xe diffusion predominantly occurs via a vacancy-mediated mechanism. Since Xe transport is closely related to diffusion of U vacancies, we have also studied the activation energy for this process. In order to best reproduce experimental data for the Xe and U activation energies, it is critical to consider the active charge-compensation mechanism for intrinsic defects in UO2±x. Due to the high thermodynamic cost of reducing U 4+ ions, any defect formation occurring at a fixed composition, i.e. no change in UO2±x stoichiometry, always avoids such reactions, which, for example, implies that the ground-state configuration of an O Frenkel pair in UO2 does not involve any explicit local reduction (oxidation) of U ions at the O vacancy (interstitial).

Effect of additives on self-diffusion and creep of UO 2

“…Table 1 shows a selected list of values calculated by various density functional theory approximations or DFTs (except [46], which uses the Mott-Littleton approximation). From this table, we see that the GGA result in [47] agrees well with the best estimate value recommended by Matzke, whereas the LDA + U result of Andersson et al [50] conforms with the aforementioned experimentally determined result taken from [54,20]. Nevertheless, for H F , there is a range of values, from 3.6 to 4.9 eV, resulting from various computations.…”

Impact of homogeneous strain on uranium vacancy diffusion in uranium dioxide

“…To begin to re-address this complex problem in UO 2 , we attempt to understand the diffusion of uranium vacancies as a function of temperature, stress state of the fuel and their interaction with the dislocations. In our simulation approach, we make use of Density Functional Theory (DFT) calculations [20][21][22] to accurately calculate the migration barriers of the single and di-vacancies of uranium as function of their charge state. This permits us to address uranium vacancy diffusion not only in UO 2 but also in UO 2+x , where, in addition to the charged vacancy (dominant in UO 2 ), neutral and di-vacancies are also important.…”

Development of Xe and Kr empirical potentials for CeO2, ThO2, UO2and PuO2, combining DFT with high temperature MD

“…Where possible, comparison has also been made to literature UO 2 DFT results [14,44,45,46]. Figure 4 shows similar trends for the trapping energies for Xe and Kr defects in CeO 2 , ThO 2 , UO 2 and PuO 2 between the empirical potential and the DFT methods used in this paper.…”

“…DFT results (partially filled columns) are reported from the literature [14,44,45,46] and the present study (using Table 2) for comparison with the new empirical potential potential (solid columns).…”

“…There has been a strong focus on investigating the impact of fission gas on thermal conductivity [6], fission gas mobility in the bulk lattice or at extended defects [3,4,5,13,14] and the behaviour of fission gas bubbles [15,16,17]. By understanding and predicting these processes over a range of conditions, in particular a large temperature range, a greater understanding of fuel behaviour can be achieved.…”