Reaction sequence and kinetics of uranium nitride decomposition.

Area of Science:

• Materials Science
• Solid State Chemistry
• Chemical Kinetics

Background:

• Uranium nitrides are crucial nuclear materials.
• Understanding their thermal decomposition is vital for nuclear fuel processing and safety.
• Previous studies lacked detailed kinetic and mechanistic insights into the UN(2)/alpha-U(2)N(3) to UN transformation.

Purpose of the Study:

• To elucidate the reaction mechanism and kinetics of uranium dinitride/uranium sesquinitride thermal decomposition to uranium mononitride.
• To quantify the nitrogen removal process during thermal decomposition.
• To determine the activation energy for this transformation.

Main Methods:

• High-temperature X-ray Diffraction (XRD) was used to monitor phase changes and lattice parameter variations.
• Electron density calculations via charge-flipping technique on XRD data visualized nitrogen content.
• Thermal decomposition experiments were conducted under an inert argon atmosphere.

Main Results:

• Lattice parameter of UN(2)/alpha-U(2)N(3) increased with temperature, indicating nitrogen removal.
• Complete decomposition of UN(2) to alpha-U(2)N(3) occurred at 675°C.
• Uranium mononitride (UN) formed after partial decomposition of alpha-U(2)N(3) at 975°C.
• The activation energy for the decomposition of UN(2)/alpha-U(2)N(3) to UN was determined as 423.8 ± 0.3 kJ/mol.

Conclusions:

• The study provides the first experimental determination of the activation energy for the thermal decomposition of uranium dinitride/uranium sesquinitride to uranium mononitride.
• Electron density calculations offer a novel method to visualize nitrogen depletion during thermal decomposition.
• The findings contribute to a deeper understanding of uranium nitride phase transformations under relevant conditions.