A classical molecular dynamics simulation of cation diffusion in isolated crystals (U_xPu_yTh_1−x−y)O₂ bounded by a free surface was performed. It was shown that in the bulk of the same model crystallite, the diffusion coefficients of cations of all types were practically identical. At the same time, the cation diffusion coefficients changed with the melting temperature of nanocrystals, which increased with increasing thorium content. At a given temperature, the diffusion coefficients were the higher, the lower were the melting points of the (U_xPu_yTh_1−x−y)O₂ crystallites. The temperature dependences of the diffusion coefficients in crystallites of different compositions converged when using coordinates normalized to the melting points.

Classical molecular dynamics simulation of cation diffusion in isolated crystals (UxPuyTh1xy)O2 bounded by a free surface was performed. It was shown that in the bulk of the same model crystallite the diffusion coefficients of cations of all types were practically identical. At the same time, the cation diffusion coefficients changed with the melting temperature of nanocrystals, which increased with increasing thorium content. At a given temperature, the diffusion coefficients were the higher, the lower were the melting points of the (UxPuyTh1xy)O2 crystallites. The temperature dependences of the diffusion coefficients in crystallites of different compositions converged when using coordinates normalized to the melting points.

Oxygen diffusion in (Pu_xTh_1−x)O₂ mixed oxide crystals was investigated using molecular dynamics simulation. The model systems were isolated nanocrystals of 5460 and 15960 particles, featuring a free surface. The oxygen diffusion coefficient D increased with decreasing thorium content, in accordance with the decrease in the melting temperature of (Pu_xTh_1−x)O₂ as x varied from 0 to 1. The temperature dependences D(T) exhibited non-linearity in the Arrhenius coordinates lnD = f(1/kT). The three linear segments of the plots corresponded to the superionic state, a transitional region, and the low-temperature crystalline phase. The transitional region was characterized by maximum values of the effective diffusion activation energy E_D(PuO₂) = 3.47 eV, E_D(ThO₂) = 5.24 eV and a complex collectiv e mechanism of oxygen migration, which involved the displacement of anions into interstitial sites . At lower temperatures, an interstitialcy mechanism of oxygen diffusion was observed. T he temperature dependence of D(PuO₂) showed quantitative agreement with low-temperature experimental data.

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.