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MATERIAL SCIENCE
ArticleName Influence of formation of uranium dioxide crystal structure defects on its thermophysical properties
DOI 10.17580/tsm.2015.08.10
ArticleAuthor Gubin S. A., Maklashova I. V.
ArticleAuthorData

National Research Nuclear University “MEPhI” (Moscow Engineering Physics Institute), Moscow, Russia:

S. A. Gubin, Professor, Head of a Chair No. 4 “Chemical Physics”
I. V. Maklashova, Senior Lecturer of a Chair No. 4 “Chemical Physics”, e-mail: ivmaklashova@mephi.ru

Abstract

During the power plant exploitation, UO2-containing fuel pellets are subjected to high temperatures and pressure, irradiation and mechanical stress. Uranium dioxide can be appeared at high pressures and temperatures in the case of accidents, such as compression in shock wave or high-speed collision. In such conditions, an intensive defect formation of crystal structure is formed. Therefore it is necessary to predict the phase states and thermophysical parameters of uranium dioxide in a wide range of pressure and temperature changes, including extreme conditions, taking into account the defect and deviations from stoichiometric composition of UO2+x. Equations of state (EOS) in the Mie-Grunaisen form for solid and liquid UO2 are obtained taking into account the formation of crystal structure defects, deviations from stoichiometry and thermal excitation energy of electrons. It is shown that amount of defects is increased with increasing of temperature and pressure. Thermophysical and thermodynamic properties of UO2 phases are calculated in a wide range of pressures and temperatures. Accounting for defect formation and electronic excitation can correctly describe the experimentally obtained accelerated growth of thermal expansion coefficient and specific heat of solid UO2 up to the melting point. Correct account of defect formation is required for reliable scaling of temperature and pressure of melting or polymorphic transformation of UO2 under compression in shock waves. In this case, the crystal structure and energy of compressed uranium dioxide are changed in the shock wave. Otherwise it is impossible to distinguish the polymorphic transformation from accumulation of defects in material crystalline structure. The resulting EOSs allow to estimate the parameters of the shock wave compression and melting of uranium dioxide in the wide range of pressures and temperatures.

keywords Equation of state, shock-wave compression, crystal defects, uranium dioxide, thermophysical properties, thermodynamic potential, phase transition
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