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A molecular dynamics simulation of the self-diffusion of thorium cations in ThO2 nanocrystals with a free surface is conducted. It is shown that a vacancy in the bulk of a crystal initiates a complex collective movement of cations, in which the interstitial positions surrounding the vacancy are involved. The diffusion coefficients of cations in the temperature range are calculated. The effective activation energy of diffusion is ED = (7.6 ± 0.7) eV. [ABSTRACT FROM AUTHOR]

Using a global optimization method, the parameters of a pair potential of the interaction of thorium and oxygen ions in ThO2 are fitted, which are compatible with the potential set MOX-07 previously proposed for UO2 and PuO2 crystals. The potential is expressed in the form U(R) = exp(X–Y•R) eV, and the optimal parameter values are X = 8.073687, Y = 3.349 Å−1. Using these parameter values, the potential reproduces the lattice constant of the ThO2 crystal in the entire temperature range up to melting, and quantitatively describes the high-temperature peak in the heat capacity that is associated with the superionic transition. [ABSTRACT FROM AUTHOR]

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]

A model of uranium hexafluoride is suggested that is based on the empirical pair potentials of U-U, F-F, U-F used for both intra- and intermolecular interactions. The potentials for this model are obtained from the lattice parameters and the thermal expansion coefficient of UF6 crystal using the molecular dynamics simulation under the periodic boundary conditions with constant volume and temperature. Within the framework of the model, the thermal expansion and sublimation of crystalline UF6 are investigated. A set of potential parameters is identified that provides satisfactory simulation of both UF6 crystal and the dependence of the UF6 saturated vapor pressure on temperature. [ABSTRACT FROM AUTHOR]