Oxygen Defect Configurations in Single-Phase UO_2.15.

Area of Science:

• Materials Science
• Solid State Chemistry
• Nuclear Materials

Background:

• Understanding the U/O phase diagram is crucial for nuclear fuel performance.
• Hyperstoichiometric uranium dioxide (UO2+x) exhibits complex defect structures.
• Previous models struggled to accurately describe the local atomic arrangement in UO2+x.

Purpose of the Study:

• To characterize the high-temperature structure of hyperstoichiometric UO2.15.
• To investigate the local atomic arrangement and defect configurations within the UO2+x phase.
• To refine structural models for oxygen defects in uranium dioxide.

Main Methods:

• Neutron total scattering experiments were performed at high temperatures.
• Analysis of diffraction data, including pair distribution functions (PDFs).
• Fitting various structural models, including defect cluster configurations, to experimental data.

Main Results:

• A single-phase fluorite structure was confirmed at high temperatures for UO2.15.
• Standard structural models did not adequately fit the short-range PDF data.
• Models incorporating oxygen defect clusters provided a better representation of local atomic arrangements.
• Moderately distorted oxygen cuboctahedra, predicted by molecular dynamics, showed the best fit.

Conclusions:

• The local atomic arrangement in UO2.15 is best described by specific oxygen defect clusters.
• Distorted oxygen cuboctahedra are a key feature of the local structure in the UO2+x phase.
• These findings advance the understanding of defect chemistry in uranium dioxide nuclear fuels.