High-precision molecular dynamics simulations were made of mixed actinide-oxide fuel, within the approximation of rigid ions and pair interactions, using high-performance graphics processors. A study was made of the self-diffusion of oxygen anions in uranium dioxide using 10 popular sets of interatomic pair potentials under periodic and isolated boundary conditions. The wide range of measured diffusion coefficients (from 10-3cm2/s at the melting point down to 10-12cm2/s at 1400K) made possible a direct comparison (without extrapolation) of simulation results with experimental data which were known only at low temperatures (<1500K). A highly detailed (1K temperature steps) calculation of the diffusion coefficient permitted plotting of the temperature dependences of the diffusion activation energy and its derivative. Both exhibited a wide (∼1000K) superionic transition region, confirming broad observed λ-peaks of heat capacity. It was shown that anion self-diffusion in model crystals without surface, or artificially embedded, defects occurred via an exchange mechanism rather than interstitial or vacancy mechanisms. The activation energy for exchange diffusion coincided with the anti-Frenkel defect formation energy calculated using lattice statics.

High-Precision Molecular Dynamics Simulation of UO2-PuO2: Anion Self-Diffusion in UO2. Potashnikov, S.I., Boyarchenkov, A.S., Nekrasov, K.A., Kupryazhkin, A.Y.: Journal of Nuclear Materials, 2013, 433[1-3], 215-26