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Our series of articles is devoted to high-precision molecular dynamicssimulation of mixed actinide-oxide (MOX) fuel in the approximation of rigidions and pair interactions (RIPI) using high-performance graphics processors(GPU). In this article we study self-diffusion mechanisms of oxygen anions inuranium dioxide (UO2) with the ten recent and widely used sets of interatomicpair potentials (SPP) under periodic (PBC) and isolated (IBC) boundaryconditions. Wide range of measured diffusion coefficients (from 10^-3 cm^2/s atmelting point down to 10^-12 cm^2/s at 1400 K) made possible a directcomparison (without extrapolation) of the simulation results with theexperimental data, which have been known only at low temperatures (T < 1500 K).A highly detailed (with the temperature step of 1 K) calculation of thediffusion coefficient allowed us to plot temperature dependences of thediffusion activation energy and its derivative, both of which show a wide(~1000 K) superionic transition region confirming the broad lambda-peaks ofheat capacity obtained by us earlier. It is shown that regardless of SPP theanion self-diffusion in model crystals without surface or artificially embeddeddefects goes on via exchange mechanism, rather than interstitial or vacancymechanisms suggested by the previous works. The activation energy of exchangediffusion turned out to coincide with the anti-Frenkel defect formation energycalculated by the lattice statics.

High-precision molecular dynamics simulation of UO2-PuO2: Anion self-diffusion in UO2