Hydride ion formation in stoichiometric UO2

Area of Science:

• Materials Science
• Nuclear Engineering
• Computational Chemistry

Background:

• Understanding atomic hydrogen behavior in uranium dioxide (UO2) is crucial for nuclear fuel performance.
• Defects and impurities significantly influence hydrogen solubility and diffusion in UO2.

Purpose of the Study:

• To investigate the energetic preferences and diffusion pathways of atomic hydrogen in UO2.
• To determine the preferred charge state and formation energy of hydrogen species in UO2.

Main Methods:

• Density Functional Theory (DFT) calculations were employed to model atomic hydrogen interactions within the UO2 lattice.
• Calculations focused on the relative energies of hydrogen as a hydride ion versus incorporated into a hydroxyl group.

Main Results:

• Hydrogen energetically favors existing as a hydride ion (H-) over forming a hydroxyl group (OH) by 0.27 eV.
• The activation energy for the conversion of the hydride state to the hydroxyl state was calculated to be 0.94 eV.
• Hydrogen's charge state is predicted to change during diffusion within the UO2 lattice.

Conclusions:

• Atomic hydrogen solubility in UO2 is governed by its preference for the hydride ion state.
• The calculated activation energy provides insight into the kinetics of hydrogen speciation changes during diffusion.
• These findings contribute to a more accurate modeling of hydrogen transport and its effects in nuclear fuels.