Cs diffusion mechanisms in UO₂ investigated by SIMS, TEM, and atomistic simulations

Area of Science:

• Materials Science
• Nuclear Engineering
• Physical Chemistry

Background:

• Understanding cesium diffusion in uranium dioxide (UO2) is crucial for nuclear fuel performance and safety.
• High-temperature behavior of fission products like cesium within the nuclear fuel matrix requires detailed investigation.

Purpose of the Study:

• To investigate cesium diffusion mechanisms in UO2 at high temperatures.
• To correlate experimental diffusion data with atomistic simulation results.
• To elucidate the role of uranium vacancy defects in cesium transport.

Main Methods:

• Experimental: 133Cs implantation in UO2, high-temperature annealing (1300-1600°C), and secondary ion mass spectrometry (SIMS) depth profiling.
• Atomistic Simulations: Nudged elastic band (NEB) method to calculate activation energies for various diffusion paths and defect types.
• Molecular Dynamics (MD) simulations to identify preferential cesium trajectories.

Main Results:

• Experimental diffusion coefficients yielded an activation energy of 1.8 ± 0.2 eV for cesium in UO2 between 1300-1600°C.
• NEB simulations predicted activation energies for Cs diffusion ranging from 0.49 to 2.34 eV, dependent on uranium vacancy defect type and concentration.
• MD simulations identified specific pathways for cesium migration, consistent with experimental findings.

Conclusions:

• Cesium diffusion in UO2 is strongly influenced by the presence and characteristics of uranium vacancy defects.
• A combination of experimental and atomistic simulation techniques provides a comprehensive understanding of high-temperature cesium transport in UO2.
• The findings contribute to improved modeling of nuclear fuel behavior and safety assessments.