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Average structure and local configuration of excess oxygen in UO(2+x).

Wang J, Ewing RC, Becker U - Sci Rep (2014)

Bottom Line: We demonstrate that the Willis cluster is a fair representation of the numerical ratio of different interstitial O atoms; however, the model does not represent the actual local configuration.The simulations show that the average structure of UO(2+x) involves a combination of defect structures including split di-interstitial, di-interstitial, mono-interstitial, and the Willis cluster, and the latter is a transition state that provides for the fast diffusion of the defect cluster.The results provide new insights in differentiating the average structure from the local configuration of defects in a solid and the transport properties of UO(2+x).

View Article: PubMed Central - PubMed

Affiliation: 1] Department of Geology and Geophysics, Center for Computation and Technology, Louisiana State University, Baton Rouge, Louisiana 70803-0001, USA [2] Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, Michigan 48109-1005, USA.

ABSTRACT
Determination of the local configuration of interacting defects in a crystalline, periodic solid is problematic because defects typically do not have a long-range periodicity. Uranium dioxide, the primary fuel for fission reactors, exists in hyperstoichiometric form, UO(2+x). Those excess oxygen atoms occur as interstitial defects, and these defects are not random but rather partially ordered. The widely-accepted model to date, the Willis cluster based on neutron diffraction, cannot be reconciled with the first-principles molecular dynamics simulations present here. We demonstrate that the Willis cluster is a fair representation of the numerical ratio of different interstitial O atoms; however, the model does not represent the actual local configuration. The simulations show that the average structure of UO(2+x) involves a combination of defect structures including split di-interstitial, di-interstitial, mono-interstitial, and the Willis cluster, and the latter is a transition state that provides for the fast diffusion of the defect cluster. The results provide new insights in differentiating the average structure from the local configuration of defects in a solid and the transport properties of UO(2+x).

No MeSH data available.


Defect models for oxygen interstitials in UO 2+x.The Willis defect cluster (a) serves as a transition state for rapid diffusion of the split di-interstitial defect (b). The latter can also migrate through a di-interstitial (c). Di-interstitial can dissociate to two immobile mono-interstitials (d).
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f4: Defect models for oxygen interstitials in UO 2+x.The Willis defect cluster (a) serves as a transition state for rapid diffusion of the split di-interstitial defect (b). The latter can also migrate through a di-interstitial (c). Di-interstitial can dissociate to two immobile mono-interstitials (d).

Mentions: In order to reconcile these computational results with the early neutron diffraction data, average occupancy numbers and displacements of O' and O” interstitials were calculated over the trajectory for each composition and are listed in Table 1. For UO2.06, at low temperatures, the interstitials are mainly displaced along <111>. As the temperature increases, the O'/O” ratio increases. As expected, the displacements increase with temperature as well. As x increases from 2.06, 2.13, to 2.19 at 2000 K, the O'/O” ratio increases because the interstitial O atoms joining neighboring di-interstitial clusters are mainly displaced along <110>. The calculated ratios of O' and O” occupancies for UO2.13, 0.15:0.09, is consistent with the experimental values of 0.08–0.33:0.10–0.16 for UO2.11–UO2.13 based on neutron diffraction345. The calculated displacements for UO2.13 are 0.77 ± 0.21 Å and 0.92 ± 0.20 Å along <110> and <111>, respectively, as compared with experimental values of 0.85 ± 0.08 Å and 1.04 ± 0.10 Å for UO2.12 at 1073 K4. Note that the calculations were done at a higher temperature for the composition, which is necessary to have adequate statistical averages in a short MD simulation. However, the temperature only has a small effect on the values of both the O'/O” ratio and displacements as the temperature increases from 1200 K to 2000 K as shown in Table 1 for UO2.06. The lower temperature is comparable to the experimental temperature conducted at 1073 K4. The probability distributions of the angle between <111> and the displacement direction of interstitial oxygen atom are shown in Figure S2. The calculated result suggests that the 2:2:2 Willis cluster model for UO2.11–13 does account for the numerical fraction of the O' and O” interstitials. However, the Willis model does not represent the local defect configuration of defect clusters. The often assumed Willis 2:2:2 defect configuration is, in fact, a transition state for the migration of a split di-interstitial cluster in the hyperstoichiometric UO2. A careful review of all the trajectories shows that the average structure of UO2+x involves a combination of defect structures including the split di-interstitial, di-interstitial, mono-interstitial, and Willis cluster, and the latter serves as a transition state for a fast diffusion of the defect cluster (Figure 4). Spectroscopic techniques such as vibrational spectroscopy can be used to validate local structure configuration of different types of defect clusters, and property measurements such as electron and ionic conductivity can be used to test if the charge transport between U4+ and U5+ is activated at a lower temperature than the oxygen migration.


Average structure and local configuration of excess oxygen in UO(2+x).

Wang J, Ewing RC, Becker U - Sci Rep (2014)

Defect models for oxygen interstitials in UO 2+x.The Willis defect cluster (a) serves as a transition state for rapid diffusion of the split di-interstitial defect (b). The latter can also migrate through a di-interstitial (c). Di-interstitial can dissociate to two immobile mono-interstitials (d).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3958918&req=5

f4: Defect models for oxygen interstitials in UO 2+x.The Willis defect cluster (a) serves as a transition state for rapid diffusion of the split di-interstitial defect (b). The latter can also migrate through a di-interstitial (c). Di-interstitial can dissociate to two immobile mono-interstitials (d).
Mentions: In order to reconcile these computational results with the early neutron diffraction data, average occupancy numbers and displacements of O' and O” interstitials were calculated over the trajectory for each composition and are listed in Table 1. For UO2.06, at low temperatures, the interstitials are mainly displaced along <111>. As the temperature increases, the O'/O” ratio increases. As expected, the displacements increase with temperature as well. As x increases from 2.06, 2.13, to 2.19 at 2000 K, the O'/O” ratio increases because the interstitial O atoms joining neighboring di-interstitial clusters are mainly displaced along <110>. The calculated ratios of O' and O” occupancies for UO2.13, 0.15:0.09, is consistent with the experimental values of 0.08–0.33:0.10–0.16 for UO2.11–UO2.13 based on neutron diffraction345. The calculated displacements for UO2.13 are 0.77 ± 0.21 Å and 0.92 ± 0.20 Å along <110> and <111>, respectively, as compared with experimental values of 0.85 ± 0.08 Å and 1.04 ± 0.10 Å for UO2.12 at 1073 K4. Note that the calculations were done at a higher temperature for the composition, which is necessary to have adequate statistical averages in a short MD simulation. However, the temperature only has a small effect on the values of both the O'/O” ratio and displacements as the temperature increases from 1200 K to 2000 K as shown in Table 1 for UO2.06. The lower temperature is comparable to the experimental temperature conducted at 1073 K4. The probability distributions of the angle between <111> and the displacement direction of interstitial oxygen atom are shown in Figure S2. The calculated result suggests that the 2:2:2 Willis cluster model for UO2.11–13 does account for the numerical fraction of the O' and O” interstitials. However, the Willis model does not represent the local defect configuration of defect clusters. The often assumed Willis 2:2:2 defect configuration is, in fact, a transition state for the migration of a split di-interstitial cluster in the hyperstoichiometric UO2. A careful review of all the trajectories shows that the average structure of UO2+x involves a combination of defect structures including the split di-interstitial, di-interstitial, mono-interstitial, and Willis cluster, and the latter serves as a transition state for a fast diffusion of the defect cluster (Figure 4). Spectroscopic techniques such as vibrational spectroscopy can be used to validate local structure configuration of different types of defect clusters, and property measurements such as electron and ionic conductivity can be used to test if the charge transport between U4+ and U5+ is activated at a lower temperature than the oxygen migration.

Bottom Line: We demonstrate that the Willis cluster is a fair representation of the numerical ratio of different interstitial O atoms; however, the model does not represent the actual local configuration.The simulations show that the average structure of UO(2+x) involves a combination of defect structures including split di-interstitial, di-interstitial, mono-interstitial, and the Willis cluster, and the latter is a transition state that provides for the fast diffusion of the defect cluster.The results provide new insights in differentiating the average structure from the local configuration of defects in a solid and the transport properties of UO(2+x).

View Article: PubMed Central - PubMed

Affiliation: 1] Department of Geology and Geophysics, Center for Computation and Technology, Louisiana State University, Baton Rouge, Louisiana 70803-0001, USA [2] Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, Michigan 48109-1005, USA.

ABSTRACT
Determination of the local configuration of interacting defects in a crystalline, periodic solid is problematic because defects typically do not have a long-range periodicity. Uranium dioxide, the primary fuel for fission reactors, exists in hyperstoichiometric form, UO(2+x). Those excess oxygen atoms occur as interstitial defects, and these defects are not random but rather partially ordered. The widely-accepted model to date, the Willis cluster based on neutron diffraction, cannot be reconciled with the first-principles molecular dynamics simulations present here. We demonstrate that the Willis cluster is a fair representation of the numerical ratio of different interstitial O atoms; however, the model does not represent the actual local configuration. The simulations show that the average structure of UO(2+x) involves a combination of defect structures including split di-interstitial, di-interstitial, mono-interstitial, and the Willis cluster, and the latter is a transition state that provides for the fast diffusion of the defect cluster. The results provide new insights in differentiating the average structure from the local configuration of defects in a solid and the transport properties of UO(2+x).

No MeSH data available.