In fluorite-type oxides, an oxygen-to-metal ratio (O/M ratio) of >2.00 indicates an oxygen-excess state with interstitial oxygen formation, whereas an O/M ratio of <2.00 indicates an oxygen-deficient state with oxygen vacancy formation. Uranium dioxide (UO₂) and U_0.7Pu_0.3O₂ can be in both oxygen-excess and oxygen-deficient states, while cerium dioxide (CeO₂) and plutonium dioxide (PuO₂) can be only in an oxygen-deficient state. Therefore, the data points for CeO₂ and PuO₂ are plotted in regions where the O/M ratio is <2.00 and the data points for UO₂ and U_0.7Pu_0.3O₂ are plotted in both regions.

Uranium dioxide (UO₂) and plutonium dioxide (PuO₂) are used as fuels in light-water reactors and fast reactors, and the diffusion of their constituent elements is closely related to redox, sintering, and element migration during irradiation; understanding these three phenomena is important with respect to the fabrication and use of these fuels. In addition to these phenomena, oxygen diffusion depends on fundamental properties, such as oxygen potential and defect concentration. Therefore, this research focuses on the oxygen diffusion in the redox behavior of oxide fuels. A relationship between oxygen defect concentration and oxygen diffusion coefficient has been formulated by analyzing experimental data on oxygen diffusion coefficients for UO₂, PuO₂, CeO₂, and (U, Pu)O₂ with fluorite-type crystal structures.

Fig. 1 shows the relationship between the oxygen self-diffusion coefficient and oxygen-to-metal ratio (O/M ratio) ^* for UO₂, PuO₂, CeO₂, and U_0.7Pu_0.3O₂. The oxygen self-diffusion coefficient for any oxide decreases as it approaches the stoichiometric composition (O/M ratio = 2.00). For UO₂ and U_0.7Pu_0.3O₂, the oxygen self-diffusion coefficient is at a minimum near their stoichiometric composition; however, the oxygen self-diffusion coefficients for the compositions on either side of this minimum are considerably different. These results provide quantitative information about the dependence of the oxygen self-diffusion coefficient on the O/M ratio for oxide fuels and thereby help enable high-accuracy prediction of the oxidation and reduction rates during fuel fabrication and of the oxygen migration behavior during irradiation.

 

^* Oxygen defect concentration and oxygen-to-metal ratio (O/M ratio)
The site fraction of oxygen point defects in a crystal is defined as the oxygen defect concentration. This concentration is directly related to the deviation from the stoichiometric composition that can be calculated from the O/M ratio. Therefore, the relationship between the oxygen self-diffusion coefficient and the O/M ratio shown in the text can be interpreted as the relationship between the oxygen self-diffusion coefficient and the oxygen defect concentration.

Kato, M., Watanabe, M. et al., Oxygen Diffusion in the Fluorite-Type Oxides CeO₂, ThO₂, UO₂, PuO₂, and (U, Pu)O₂, Frontiers in Nuclear Engineering, vol.1, 2023, 1081473, 10p.

Paper URL: https://doi.org/10.3389/fnuen.2022.1081473

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