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

Area of Science:

• Materials Science
• Solid-State Physics
• Nuclear Engineering

Background:

• Determining the local configuration of interacting defects in crystalline solids is challenging due to the lack of long-range periodicity.
• Uranium dioxide (UO2+x), a key nuclear fuel, exhibits hyperstoichiometry with partially ordered interstitial oxygen defects.
• The established Willis cluster model, derived from neutron diffraction, is the prevailing explanation for UO2+x defect structure.

Purpose of the Study:

• To investigate the accuracy of the Willis cluster model in representing the local configuration of interstitial oxygen defects in UO2+x.
• To elucidate the average structure and defect dynamics in hyperstoichiometric uranium dioxide using advanced simulation techniques.

Main Methods:

• First-principles molecular dynamics (MD) simulations were employed to model the defect structures and dynamics in UO2+x.
• Analysis focused on comparing simulation results with the predictions of the Willis cluster model.

Main Results:

• While the Willis cluster accurately predicts the numerical ratio of interstitial oxygen atoms, it fails to represent the actual local atomic configuration.
• MD simulations reveal that the average structure of UO2+x comprises a mixture of defect types: split di-interstitial, di-interstitial, mono-interstitial, and the Willis cluster.
• The Willis cluster is identified as a transient state facilitating rapid diffusion of defect clusters within the UO2+x lattice.

Conclusions:

• The study highlights the critical distinction between average crystal structure and local defect configuration in solids.
• The findings challenge the long-held Willis cluster model, proposing a more complex, multi-component defect structure for UO2+x.
• New insights into the transport properties of UO2+x are provided, emphasizing the role of defect cluster dynamics.