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A molecular dynamics simulation of small PuO2 nanocrystals consisting of 105−174 ions at the temperature T = 1700 K was carried out. The structures of the vertices and the surface created during thermal disordering and subsequent relaxation of these nanocrystals were studied. The vertices formed by two adjacent plutonium cations, as well as truncated vertices of the regular octahedron that consisted of 4 plutonium cations lying in the same plane (100), were most frequently encountered. Sharp vertices of the regular octahedron ending in one cation usually corresponded to metastable states of the crystal lattice or to crystallites with an increased specific energy. According to the results, sequences of the “magic numbers” were proposed to specify the sizes of plutonium dioxide nanocrystals characterized by the most stable surface. [ABSTRACT FROM AUTHOR]

A molecular dynamics simulation of the melting of UO2 nanocrystals of the optimal truncated octahedron shape in the size range from 4317 to 381174 ions was carried out. The dependence of the melting temperature of the nanocrystals on the size was obtained at the time of evolution of the model system up to 150 ns. This dependence was characterized by nonlinearity caused by a change in the melting mechanism corresponding to the transition from larger nanocrystals to smaller ones. The nanocrystals containing more than 20 000 ions undergo the phase transition as a result of the formation of the surface melt on a single (111) face, which caused the irreversible propagation of the melt into the bulk. Smaller crystallites could melt at temperatures below the crystallization temperature of the (111) face, provided that two adjacent faces melt simultaneously due to the thermal activation. The difference in melting temperatures of the largest and smallestmodel crystals was 320K, which corresponds to the experimental estimates. [ABSTRACT FROM AUTHOR]

The process of mass transfer in (U,Pu)O2 nanocrystals is studied using the molecular dynamics simulation. The dependence of the vacancy concentration on temperature was estimated by calculating the time of presence of the vacancies in the crystallite bulk. The effective formation energy of vacancies obtained from this dependence was (10 ± 2) eV. Within the limits of error, this energy coincides with the formation energy of the Schottky defect calculated by the lattice static method, and also with the effective energy characterizing the frequency of vacancy formation. [ABSTRACT FROM AUTHOR]

Molecular dynamic simulation of krypton bulk diffusion in stoichiometric UO2 nanocrystals with free surface is carried out. The values of the diffusion coefficient in the temperature range from 1000 K to 3000 K are obtained. The magnitudes of the diffusion coefficient varied from 3.10-8 to 1.10-4 cm²/s. It is shown that the temperature dependence of the diffusion coefficient could be characterized by a constant activation energy ED = (1.00 - 0.03) eV. The predominant mechanism of diffusion consisted in the movement of the atom by the interstitial positions via the sites of the anion sublattice, accompanied by the displacement of oxygen ions to the interstitial positions. [ABSTRACT FROM AUTHOR]