Hydrothermal Conversion of Uranium(IV) Oxalate into Oxides: A Comprehensive Study

Area of Science:

• Materials Science
• Nuclear Engineering
• Inorganic Chemistry

Background:

• Advanced oxide fuels are crucial for future nuclear reactors.
• Wet chemistry routes offer promising fabrication methods for these fuels.

Purpose of the Study:

• Investigate the hydrothermal conversion of uranium(IV) oxalate to uranium oxides.
• Determine the effects of temperature, pH, and kinetics on oxide properties.
• Optimize conditions for producing pure uranium oxides with desired characteristics.

Main Methods:

• Multiparametric study of hydrothermal conversion.
• Uranium M4-edge High-Energy-Resolution Fluorescence-Detected X-ray Absorption Near Edge Structure (HERFD-XANES) experiments.
• X-ray Diffraction (XRD) analysis.
• Morphological characterization via pH tuning.

Main Results:

• Hydrothermal conversion at 180-200 °C and pH ≤ 1 yielded UO2+/U4O9 mixtures (O/U ratio 2.38 ± 0.10).
• Higher temperatures (220-250 °C) reduced the O/U ratio to 2.13 ± 0.04.
• pH influenced morphology: bipyramidal aggregates (pH ≤ 1), microspheres (pH 2-5), and nanometric powders (pH > 5).
• Efficient elimination of carbon and water occurred with increasing temperature.
• Kinetics study indicated oxide formation within hours, with an initial U(VI)-bearing amorphous phase reducing over time.

Conclusions:

• Hydrothermal conversion is an effective method for producing pure uranium oxides.
• Temperature and pH are critical parameters controlling stoichiometry and morphology.
• The process likely involves a two-step mechanism: oxidative decomposition followed by reduction and hydrolysis.
• This route offers an efficient pathway for synthesizing uranium oxide powders for nuclear applications.