Formation of ionic networks in molten salt mixtures. Computer experiment

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Abstract

Molten salts are used as heat-conducting media in liquid salt reactors and solar installations. Knowledge of the thermal conductivity of molten salt is necessary for the safe operation of these units. Computational methods are an alternative way to the hard-to-reach experimental way of determining thermal conductivity. In this work, the temperature dependence of the thermal conductivity of the molten salt FLiNaK, as well as this molten salt with NdF3 dissolved in it, was calculated using the method of equilibrium molecular dynamics. The temperature trend of thermal conductivity, as well as its change after the dissolution of NdF3 in FLiNaK, is explained based on the determination of the dynamic network of ionic bonds that exists in the molten salt model. Networks of ionic bonds were established with an upper limit of interionic distance of 0.2 nm for both types of salt melts and with a limit of 0.27 nm for the Nd–F network in a melt containing NdF3. These networks of bonds appear in different parts of the system over time and may disappear completely. The total number of dynamic network nodes, determined during the correlation of heat flows, has an impact on the thermal conductivity value of the simulated system. A new method for interpreting the temperature behavior of the thermal conductivity of molten salt in a computer model can be used for predictive purposes when fluorides of various lanthanides and actinides are dissolved in salt melts.

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About the authors

А. E. Galashev

Institute of High Temperature Electrochemistry, Ural Branch of the Russian Academy of Science; Ural Federal University named after the first President of Russia B.N. Yeltsin

Author for correspondence.
Email: galashev@ihte.uran.ru
Russian Federation, Yekaterinburg; Yekaterinburg

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Supplementary files

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2. Fig. 1. General view of the FLiNaK + 15 mol% NdF3 system before the MD calculation (t = 0 s, shown on the left) and at the end of the calculation when the melt was obtained at T = 1020 K (t = 2.5 ns, shown on the right).

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3. Fig. 2. Relative number of interconnected positive and negative ions in the FLiNaK + + 15 mol% NdF3 system forming Li+-F- bonds with lengths less than 0.2 nm (a) and Nd3+-F- bonds with lengths ≤0.27 (b); dashed lines and numbers near the curves show average values of the number of interconnected Nb ions.

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4. Fig. 3. Placement of Li+ ions in the system FLiNaK + + + 15 mol% NdF3 (800 K) at the end of the calculation.

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5. Fig. 4. Nodes of the largest bond mesh with bond lengths of 0.2 nm or less between positive and negative ions in the system FLiNaK + 15 mol% NdF3 at 800 K.

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6. Fig. 5. Placement of Nd3+ ions in the FLiNaK + + 15 mol% NdF3 system at the end of the calculation at 1020 K.

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7. Fig. 6. Nodes of the bond mesh between Nd3+ and F- ions in the FLiNaK + 15 mol% NdF3 system with bond lengths of 0.27 nm or less at the end of the calculation at 1020 K.

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8. Fig. 7. Temperature dependence of the relative number of bound ions for the systems FLiNaK and FLiNaK + 15 mol% NdF3; dashed lines - linear approximation of the presented dependences; in the figure the term “total number” means that the bond lengths Lb of all positive ions with F - not exceeding 0.2 nm, as well as Lb of dissolved ions with Nd3+-F bonds not exceeding 0.27 nm are taken into account.

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9. Fig. 8. Thermal conductivity of the systems FLiNaK and FLiNaK + + + 15 mol% NdF3 determined in the molecular dynamic calculation (the present work) and in the experiment to determine the coefficient λ by the laser flash method [31].

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