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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]

A simple mathematical model of UN, U2N3, and UN2 crystals is used for reconstructing the pair potentials of interaction of the nearest neighbors and subsequent molecular dynamics simulation. The corresponding parameters of the U−N potential in U2N3 and UN2 crystals are obtained from the experimental values of the lattice constant and the bulk moduli of UN, U2N3, UN2 at room temperature. The linear expansion coefficients of U2N3 and UN2 crystals, as well as the U2N3 bulk modulus are estimated within the harmonic approximation. The dependences of the U2N3 and UN2 lattice constants on temperature are calculated by the molecular dynamics method. [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 sintering of two octahedral uranium dioxide nanocrystals contacting in the vertex region is carried out. Two main stages of the sintering process are observed. At the beginning of the simulation, the neck between the crystallites is formed due to the mutual attraction of the adjacent (111) faces. Further increase in the neck thickness occurs due to transport of the material from the surface of the crystallites in a much slower diffusion process. The estimate of the activation energy of this process is in the range from 11 to 14 eV. [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]

The interaction of xenon clusters with the collision cascades in the PuO2 crystals is investigated using the molecular dynamics simulation and the approximation of the pair interaction potentials. The potentials of interaction of Xe atoms with the surrounding particles in the crystal lattice are suggested, that are valid in the range of high collision energies. The cascades created by the recoil 235U ions formed as the plutonium α-decay product are considered and the influence of such cascades on the structure of the xenon clusters is analyzed. It is shown, that the cascade-cluster interaction leads to release of the xenon atoms from the clusters and their subsequent re-solution in the crystal bulk. [ABSTRACT FROM AUTHOR]