Abstract:Background. Uranium dioxide is a fuel employed in the majority of modern nuclear power rectors. The operation temperature of the fuel can exceed 2000K, the pressure of the gaseous fission products of uranium in the fuel core is sometimes above 100atm. This may cause fuel microstructural changes, its swelling, recrystallization, and grain sintering. It is difficult to perform a detailed experimental study of nuclear fuel properties at critical temperatures due to hard environmental conditions. Thus, to obtain the required information, the mathematical simulation is commonly used. This paper presents calculations of melting processes in uranium dioxide crystals comprising nano-scale crystals. Materials and methods. The simulation was performed by molecular dynamics method using DL_POLY code. The translated cell was chosen as a cubic crystal with the fluorite structure. Cubic crystallites were built by translating the unit cell in three directions. The periodic boundary conditions (an infinite crystal) and zero boundary conditions (a free crystal in vacuum) were used while calculating. Interatomic interaction potential was chosen in the Born-Mayer form. Some of its parameters were taken in the form of piecewise linear slowly varying temperature functions. Results. The paper describes the investigation of uranium dioxide phase transformations performed by molecular dynamics simulation. Three different methodologies were used to determine the temperature of superionic state transition and the melting point. Temperatures both for uranium dioxide macrocrystal and for cubic nanocrystals with the size of 2.2–4.4 nm were estimated. The relation of the nanocrystal size reduction to temperature decrease was registered. Conclusions. The three following methods were used for critical temperature determination: 1) the analysis of the type of radial distribution function; 2) the analysis of structural scattering factor changes; 3) the analysis of the enthalpy – temperature dependence. The structural factor method is more preferable for macrocrystals because the dependences obtained have well-defined linear regions. It simplifies a further analysis. The enthalpy method is more suitable for nanocrystals due to its higher accurancy. The results showed that the phase transition temperatures in nanocrystals of UO$_2$ decrease significantly with the reduction of the crystal size.
Keywords:molecular dynamics, uranium dioxide, melting point of nanocrystalls.
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