43 research outputs found
Chemisorption of a molecular oxygen on the UN (001) surface: ab initio calculations
The results of DFT GGA calculations on oxygen molecules adsorbed upon the
(001) surface of uranium mononitride (UN) are presented and discussed. We
demonstrate that O2 molecules oriented parallel to the substrate can dissociate
either (i) spontaneously when the molecular center lies above the surface
hollow site or atop N ion, (ii) with the activation barrier when a molecule
sits atop the surface U ion. This explains fast UN oxidation in air
High-precision molecular dynamics simulation of UO2-PuO2: Anion self-diffusion in UO2
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.Comment: 18 pages, 11 figures, 5 table