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

TL;DR

This study uses high-precision molecular dynamics simulations to analyze oxygen anion self-diffusion in UO2, revealing exchange mechanisms dominate and identifying a broad superionic transition region with detailed temperature dependence.

Contribution

It provides a comprehensive, temperature-resolved analysis of oxygen diffusion in UO2 using multiple interatomic potentials, confirming exchange mechanisms and linking activation energy to defect formation.

Findings

Exchange diffusion mechanism dominates in UO2 without defects.

Superionic transition occurs over a broad temperature range (~1000 K).

Activation energy matches anti-Frenkel defect formation energy.

Abstract

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