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

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 local spin-density approximation plus 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 trivacancy sites to incorporate xenon in hyperstoichiometric regime at high temperatures. The results show that in hypostoichiometric regime neutral trivacancy sites are the most favored position for diluted xenon gas, whereas in hyperstoichiometric 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 metastable states difficulty that frequently encountered in density functional theory plus U applications, namely, the quasiannealing procedure, is also discussed.

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

“…Due to its high impact on nuclear fuel performance the properties of Xe in UO 2±x have been extensively studied using a variety of theoretical 2,[6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] and experimental techniques [3][4][5][25][26][27][28][29][30][31] . Matzke et al [3][4][5] have published several reviews on this topic and concluded that Xe diffusion can be represented by an Arrhenius model, D = D 0 e −Ea/k b T , with activation energies (E a ) that depend on UO 2±x stoichiometry.…”

“…A related model was introduced by Geng et al in the UO 2+x regime 24 . In principle, our study could be expanded to include such effects but this exercise has been left as future work and our discussion refers to the thermodynamic model in Table I.…”

Anomalies in nonstoichiometric uranium dioxide induced by a pseudo phase transition of point defects

“…[7]. Especially, in order to eliminate electronic meta-stable states that introduced via strong correlations of 5f orbitals, we adopted the quasi-annealing method [8] to lower the electronic energy slowly by controlling the random disturbance from the ionic system which acts as a kind of heat bath. Defect populations were evaluated using point defect model [9][10][11] and presumed a physical condition in the closed regime, which corresponds to the interior of a bulk material and thus no exchange of particles with the exterior can occur.…”

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).…”