Written by Lauren Hunter, Technical Sales Specialist (BSE, EIT)
Keywords: molten salt, small modular reactor, advanced nuclear reactors, concentrated solar power, thermal energy storage, heat transfer, heat storage, generation IV, sustainable energy
Figure 1: Solid molten salts undergoing melting [1]
What are Molten Salts?
A molten salt is a salt that is solid at standard temperature and pressure but turns into a liquid when heat is added to it. Molten salts are gaining interest in many fields such as catalysis, chemistry, and especially heat transfer and heat storage [2], because of their unique thermodynamic properties such as high heat capacity, low melting point, high boiling point, and low vapour pressure. Individual salts are often blended to produce binary or ternary salts in effort to achieve desired properties such as lower melting point.
What are Molten Salts Used For?
Molten salts are applied as heat transfer and heat storage mediums in solar thermal energy storage, as well as in upcoming advanced (generation IV) nuclear reactor designs, and thus play a key role in the advancement of sustainable energy. Different molten salt blends are used for the many designs, and it is imperative to understand their unique thermophysical properties.
What are the Challenges with Characterizing Molten Salts?
Molten salts can be difficult to characterize due to their corrosiveness, hygroscopicity, and high temperature requirements. High temperature calorimetry can bring key information on the thermal properties of pure or blends of salts, like melting and crystallization temperatures, heat of melting, heat capacity and heat of mixing and/or dissolution [2]. Outlined below are examples of how Setaram thermal analysis equipment is useful for the thermal analysis of a variety of molten salts.
Heat Transport, Heat Storage, and Molten Salts
Molten Salt Nuclear Reactors
Molten salts are employed in advanced small modular nuclear reactor designs as the primary coolant medium. Since the salts have low vapour pressures and high boiling points, the risk of a loss of coolant accident is significantly reduced in comparison to current reactors which use water as a coolant [2],[3]. If the reactor were to lose control, this would cause the reaction, and thus, the heat generation to stop.
Figure 2: Schematic of molten salt reactor heat transport flow [4]
Molten salts have a large range between their melting and boiling points, allowing the reactors to operate at high temperatures and low pressures, which makes them more efficient at generating electricity and safer. Many molten salt reactor designs are being developed, all with varying blends of salts. Since molten salts are used as heat transfer material, they will be heated and cooled to various temperatures over the heat cycle. There is a need to study molten salt phase diagrams and fully understand their behaviour before commissioning these reactors.
Enthalpies of mixing for LiF-ThF4 measured with the Setaram CALVET DC with custom nickel cells [5]
The Setaram CALVET DC with HFDSC and drop sensor were used by Dr. Ondrej Benes and his team to determine the thermodynamic properties of a LiF-Th4 system for fuel salts [2],[5]. The enthalpies of mixing for LiF-Th4 and LiF-K systems were measured using the CALVET DC with custom nickel cells [2]. The CALVET DC is an isothermal or temperature scanning differential calorimeter with operating temperatures up to 1600°C and drop calorimetry capacities, great for high temperature molten salt characterization.
 Figure 4: Setaram Calvet DC [6] |
Heat flux DSC up to 1600°C – for accurate heat capacity, heat, and glass transition measurements. Drop calorimetry up to 1500°C – for accurate heat capacity, heat of dissolution and heat of formation measurements. Variety of atmosphere conditions possible with multiple carrier and reactive gas options. Convenience and economy with one instrument and furnace for TGA, TG-DSC, TGDTA, DSC, DTA, and TMA up to 1600°C. |
Solar Power Plants
Concentrated solar power plants (CSPs) use mirrors to concentrate sunlight on a target to intensely heat it. The heat is then used to power a heat engine connected to an electrical power generator. Molten salts are used to transport the solar thermal energy to the engine or store it as heat so electricity can be generated later [2]. This storage ability makes an otherwise intermittent energy source a reliable one. Nitrate and nitrite salts are the primary compounds applied in the salts at CSPs. It is important to understand how to optimize the thermal stability of these salts.
 Figure 5: Aerial view of CSP [7] |
 Figure 6: CSP with thermal storage schematic [8] |
Rene I. Olivares from CSIRO in Australia studied the thermal stability of nitrate and nitrite salts for solar energy storage using the TG/DSC+MS coupled technique [2],[9]. Temperature scanning experiments up to 1000°C in argon, nitrogen, air, and oxygen atmospheres were completed and determined that controlling the atmosphere can significantly improve the thermal stability of the salt. The maximum temperature of the experiment was between 650°C and 700°C under oxidizing atmosphere [8].
Figure 7: TG, DSC & MS data for the decomposition of the LiNO3-Na-NO3-KNO3 salt in air [9]
 Figure 8: Setaram Themys TGA [6] |
High accuracy & versatility hang-down symmetrical beam balance, specifically designed for TGA application. Ultra-high temperature capability to 2400°C with a single furnace. Modular adaptations allowing TGA only, DTA only, TG-DTA, and TMA up to 2400°C, DSC only and TG-DSC up to 1600°C all in one instrument External coupling capability designed for evolved gas analyzers (FTIR, MS, GCMS, MSFTIR, or FTIR-GCMS) |
To find out more about thermal analysis of molten salts, or if you want to learn more about Setaram products, contact us directly at sales@ctherm.com.
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Works Referenced
[1] Molten salts. DuBois. (2020). https://www.duboischemicals.com/manufacturing/products/heat-treatment/molten-salts/
[2] Thermal Analysis, calorimetry and molten salts. Setaram. (2020). AN693-Thermal-analysis-calorimetry-and-molten-salt.pdf (setaramsolutions.com)
[3] Molten Salt Reactors. World Nuclear Association. (2021). https://world-nuclear.org/information-library/current-and-future-generation/molten-salt-reactors.aspx
[4] Waldrop, M.M. (2019). Nuclear Technology Abandoned Decades Ago Might Give Us Safer, Smaller Reactors. Nuclear Technology Abandoned Decades Ago Might Give Us Safer, Smaller Reactors | Discover Magazine
[5] E. Capelli et al., J. Chem. Thermodynamics 58 (2013) 110–116.
[6] Setaram Solutions. https://setaramsolutions.com/
[7] World’s Largest Concentrated Solar Power Plant is in Dubai. HELIOSCSP. (2019). World’s Largest Concentrated Solar Power Plant is in Dubai – HELIOSCSP
[8] W. Ding et al., Solar Energy Materials and Solar Cells 193 (2019) 298-313. Molten chloride salts for next generation concentrated solar power plants: Mitigation strategies against corrosion of structural materials.
[9] R.I. Olivares, W. Edwards / Thermochimica Acta 560 (2013) 34– 42
Additional Sources
https://ctherm.com/resources/webinars/test-methods-thermal-energy-storage-materials/
About the Author
 Lauren Hunter, Technical Sales Specialist |
Lauren works in technical sales with the SETARAM product line and is working towards her professional engineer designation. She holds a Bachelor of Science in Chemical Engineering with the Nuclear Power Option from the University of New Brunswick. |