High-temperature X-ray diffraction study of uranium-neptunium mixed oxides

Area of Science:

• Materials Science
• Nuclear Chemistry
• Solid State Chemistry

Background:

• Minor actinides (Am, Np, Cm) recycling in UO2 blankets is crucial for nuclear fuel sustainability.
• Limited data exists on uranium-neptunium-oxide phase equilibria at high temperatures.
• Understanding these systems is vital for advanced nuclear fuel cycle development.

Purpose of the Study:

• To investigate the high-temperature phase equilibria of (U1-yNpy)O2 mixed oxides.
• To elucidate the phase transformations during the oxidation of uranium-neptunium oxides.
• To clarify the controversial incorporation of neptunium into the U3O8-type structure.

Main Methods:

• In situ high-temperature X-ray diffraction (HT-XRD) up to 1373 K.
• Oxidation experiments on (U1-yNpy)O2 solid solutions.
• Analysis of phase transformations and structural characterization.

Main Results:

• Neptunium stabilizes the UO2-type fluorite structure at elevated temperatures.
• The U3O8-type orthorhombic structure was observed across a broad compositional range.
• Neptunium incorporation into the U3O8-type phase was definitively demonstrated.

Conclusions:

• The tetravalent state of neptunium is key to the observed phase relationships.
• This research provides critical data for minor actinide recycling strategies.
• Findings resolve previous ambiguities regarding neptunium's behavior in U-Np-O systems.