First-principles Study of UO $$_2$$ 2 Lattice Thermal-Conductivity: A Simple Description
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RESEARCH PAPER
First-principles Study of UO2 Lattice Thermal-Conductivity: A Simple Description Samira Sheykhi1 • Mahmoud Payami1 Received: 16 May 2020 / Accepted: 2 July 2020 Ó Shiraz University 2020
Abstract Modeling the high-T paramagnetic state of bulk UO2 by a non-spin-polarized calculation and neglecting the Hubbard-U correction for the f electrons in U atoms, the lattice thermal conductivity of bulk UO2 is investigated by the exact solution of the Boltzmann transport equation for the steady-state phonon distribution function. The resulting phonon dispersion shows good agreement with experiment and when the non-analytical correction of the dynamical matrix was added, this agreement is enhanced. Analysis of the results shows that TA branches corresponding to U-atoms vibrations have the largest lifetimes and therefore have dominant role in thermal conductivity, while the optical branches corresponding mainly to O-atoms vibrations have the shortest lifetimes. Using this simple model, our results for the thermal conductivity show a very good agreement with the experiments. The calculations are repeated for bulk UO2 with different U-235 concentrations of 3%, 5%, 7% and 20%, and the results show a small decrease in thermal conductivity which arises from scattering of phonons by impurities. Keywords Uranium dioxide Lattice thermal conductivity Boltzmann transport equation Acoustic branch Optical branch Phonon lifetime Density-functional perturbation theory
1 Introduction One of the vastly used fuels in nuclear power reactors is uranium-dioxide, UO2 . The performance of a nuclear fuel is highly correlated with its thermal conductivity, and therefore, studying the thermal conductivity of nuclear fuel and understanding the mechanisms behind it is one of the most active fields of research in nuclear industry. Due to the fission processes of the uranium atoms, large amounts of heat are released, leading to large temperature gradients throughout the fuel rod. Having an efficient thermal conductivity, the generated temperature-gradients in the fuel are immediately balanced and the heat is easily extracted by the coolant so that the fuel system does not meet any safety problems because of any temperature increase or thermal stresses. & Mahmoud Payami [email protected] 1
School of Physics and Accelerators, Nuclear Science and Technology Research Institute, AEOI, P.O.Box 14395-836, Tehran, Iran
Experiments have determined the crystal structure of UO2 as a 3k-order antiferromagnet (AFM) at T\30 K, and paramagnetic at higher temperatures (Amoretti et al. 1989; Faber et al. 1975). The uranium atoms sit on the sites ˚ while of an FCC structure with lattice constant a ¼ 5:47 A, the oxygen atoms are positioned at sites with Pa3 symmetry (Idiri et al. 2004). Uranium-dioxide is electrically an insulator material (the so-called Mott insulator), and because of the localized partially filled f-orbitals on U atoms, it is a strongly correlated electron system. Theoretical d
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