Molten salt is exactly what it sounds like: salt heated past its melting point until it becomes a liquid. Table salt (sodium chloride) melts at about 800 °C, but the salts used in energy and industry are typically mixtures designed to melt at much lower temperatures, sometimes below 150 °C. What makes these liquids remarkable is how well they absorb and hold heat, which has turned them into critical working fluids for solar power plants, experimental nuclear reactors, advanced batteries, and a growing list of industrial processes that would surprise most people.
Why Liquid Salt Is So Useful
When you melt a salt, you get a liquid that can absorb enormous amounts of thermal energy per unit volume. A cubic meter of molten salt stores far more heat than the same volume of gas, and it does so at atmospheric pressure, meaning the tanks and pipes that hold it don’t need to be as thick or heavily engineered as those in a pressurized steam system. That combination of high heat capacity and low operating pressure is the reason engineers keep returning to molten salts whenever they need to move or store heat at high temperatures.
The specific salt blend matters enormously. Engineers choose mixtures based on the temperature range they need, the chemical stability of the salt over thousands of hours, and how aggressively it corrodes the metals it contacts. A mixture of sodium nitrate and potassium nitrate called “Solar Salt” is the workhorse of concentrated solar power, while fluoride-based salts like FLiBe (lithium beryllium fluoride) dominate nuclear reactor research, and chloride salts show up in both metallurgy and newer reactor designs. Researchers have measured detailed properties of these blends, including heat capacity, melting point, heat of fusion, viscosity, vapor pressure, density, and thermal conductivity, building the engineering databases that designers need to size pumps, heat exchangers, and storage tanks.1Journal of Solar Energy Engineering. Thermophysical Properties Experimentally Tested for NaCl-KCl-MgCl2 Eutectic Molten Salt as a Next-Generation High-Temperature Heat Transfer Fluids in Concentrated Solar Power Systems
Storing Sunlight as Heat
Concentrated solar power (CSP) plants use mirrors to focus sunlight onto a central receiver, heating a fluid to high temperatures and then using that heat to drive a steam turbine. Molten salt is the fluid of choice because it can soak up heat during the day and release it hours later, after the sun has set, solving one of solar energy’s core problems. The most widely deployed formulation is Solar Salt, a blend of roughly 60% sodium nitrate and 40% potassium nitrate. It melts around 220 °C and has traditionally been used up to about 565 °C, but researchers have recently pushed it further. A 100-kilogram-scale experiment demonstrated that Solar Salt can operate at temperatures up to 600 °C when synthetic air is purged through the system, with clear evidence of feasibility despite some thermal decomposition at those extremes.2Solar Energy Materials and Solar Cells. Concentrating solar power at higher limits: First studies on molten nitrate salts at 600 °C in a 100 kg-scale hot tank
Raising the upper temperature limit matters because hotter steam makes more efficient electricity. Understanding exactly where these salts start to break down is an active area of study, with researchers analyzing the decomposition kinetics of potassium nitrate, sodium nitrate, sodium nitrite, the binary Solar Salt blend, and the ternary mixture called Hitec to pin down their maximum safe working temperatures.3Energy Procedia. Effect of Heating Rates and Composition on the Thermal Decomposition of Nitrate Based Molten Salts Separate work using thermogravimetric and calorimetric methods found that under an oxidizing atmosphere, nitrite/nitrate salt mixtures can be thermally stable to between 650 °C and 700 °C, partly because oxygen converts some nitrite to nitrate, which raises the decomposition threshold even as it nudges the melting point upward.4Solar Energy. The thermal stability of molten nitrite/nitrates salt for solar thermal energy storage in different atmospheres
An alternative approach is to lower the melting point instead of raising the ceiling. A ternary mixture of lithium nitrate, potassium nitrate, and sodium nitrate melts at just 129 °C and remains thermally stable above 550 °C. That wider liquid range gives plant operators more flexibility to avoid freezing in the pipes overnight. The tradeoff is longevity: after roughly 15,000 hours of operation, researchers observed that the heat capacity of this ternary blend dropped by about 20%, and unwanted byproducts like calcium carbonate, magnesium carbonate, and lithium oxides began to appear as the mixture slowly separated.5Europe PMC. Long-Term Evaluation of a Ternary Mixture of Molten Salts in Solar Thermal Storage Systems: Impact on Thermophysical Properties and Corrosion That kind of degradation data is exactly what engineers need before committing a salt formulation to a 30-year power plant.
Fuel and Coolant for Nuclear Reactors
Molten salt reactors (MSRs) are an entirely different application with a shared principle: exploit the heat-carrying capacity of liquid salt at high temperature and low pressure. In one design, the nuclear fuel itself is dissolved directly into a fluoride salt, circulating through the reactor core as a hot liquid. In another, molten salt simply serves as the coolant for conventional solid fuel. Either way, the liquid phase stores far more heat per cubic meter than gas coolants can, and because the salt stays liquid at atmospheric pressure, the reactor vessel doesn’t need the massive pressure containment that conventional water-cooled reactors require. That translates to smaller, thinner-walled systems.6Annals of Nuclear Energy. Molten salt for advanced energy applications: A review
Safety is one of the most compelling arguments for MSRs. The fuel salt itself has a strong negative temperature coefficient of reactivity, meaning that if the reactor starts to overheat, the nuclear chain reaction naturally slows down without any operator intervention. On top of that, many MSR designs incorporate a freeze plug at the bottom of the core: a small section of salt kept solid by active cooling. If temperatures climb too high, the freeze plug melts, and gravity drains the entire fuel salt into emergency dump tanks below the reactor, shutting everything down passively.7Journal of Radiation Research and Applied Sciences. Safety assessment of molten salt reactors in comparison with light water reactors That kind of walk-away-safe behavior is difficult to achieve in conventional pressurized water reactors.
One practical concern with MSRs is tritium, a radioactive form of hydrogen that can be generated in molten fluoride salts under neutron bombardment. Because tritium permeates through metals at high temperatures, it can escape containment and pose a radiological risk. Researchers have been simulating shell-and-tube metallic permeators designed to remove tritium directly from the molten salt before it migrates elsewhere. The current bottleneck isn’t the metal’s permeability but rather the rate at which tritium molecules transfer from the salt surface to the metal surface, which means improving the permeator’s physical design could raise removal efficiency significantly.8Annals of Nuclear Energy. A simulation study of tritium removal from molten salt at high temperature with tritium permeation through metallic material
Heat-Treating Metals and Recycling Nuclear Fuel
Long before molten salt became a buzzword in clean energy, it was a mainstay of metallurgy. Hardening, carburizing, and nitrocarburizing steel are heat treatment processes commonly carried out in baths of molten salt.9Molten Salts Chemistry. Salt Bath Thermal Treating and Nitriding Dipping a steel part into a salt bath provides extremely uniform heating because the liquid makes contact with every surface simultaneously, avoiding the hot spots and cold spots that can occur in gas-fired furnaces. The result is more consistent hardness and less distortion in finished parts, which is why salt bath treatment remains common in toolmaking, automotive, and aerospace manufacturing.
A newer industrial application is the recycling of spent nuclear fuel through pyroprocessing. Conventional reprocessing uses liquid solvents at lower temperatures, but pyroprocessing dissolves spent fuel in molten chloride or fluoride salts, then uses electrochemistry to separate uranium and plutonium from fission products. The oxide components of spent fuel can be reduced to metals by chemical or electrochemical means in these salts, and uranium can then be recovered by electrorefining. In some cases, uranium dioxide can be separated from other oxides directly, skipping the reduction step entirely.10Progress in Nuclear Energy. Review Molten salt technologies for recycling spent nuclear oxide fuel The appeal is that pyroprocessing can handle fuel types that aqueous methods struggle with, and it produces a more compact waste stream.
That waste stream still needs to go somewhere. Chloride molten-salt wastes left over from pyroprocessing contain radioactive fission products that must be immobilized for long-term disposal. Two main strategies exist: direct immobilization, in which the salt is locked into a durable solid matrix such as glass-bonded sodalite, halide-tolerant glasses, or phosphate-based ceramics; and conversion-based immobilization, in which the chloride salts are first chemically converted and then incorporated into glass or ceramic forms.11E3S Web of Conferences. Environmental Treatment and Immobilization Strategies for Chloride Molten-Salt Wastes from Spent-Fuel Pyroprocessing: A Review Neither approach is trivial, and getting the waste form right is one of the gatekeepers for broader adoption of pyroprocessing.
The Corrosion Problem
For all their thermal advantages, molten salts are punishing on the metals that contain them. The dominant corrosion mechanism in both fluoride and chloride salts is the selective leaching of chromium from the alloy surface. Chromium dissolves preferentially into the salt, and the attack is especially aggressive along grain boundaries in the metal. In molten FLiBe salt at 700 °C, researchers found that the severity and mechanism of corrosion differed depending on whether the salt was contained in a graphite capsule or a metallic one, a complication that matters because MSR cores contain both materials side by side.12Journal of Electronic Materials. Corrosion of Structural Alloys in High-Temperature Molten Fluoride Salts for Applications in Molten Salt Reactors
Alloy selection is the primary defense. Hastelloy N, a nickel-based alloy originally developed for molten salt service in the 1960s, consistently outperforms alternatives. In molten potassium chloride at high temperature, Hastelloy N corroded at about 35 micrometers per year, compared with roughly 53 for Alloy 600 and over 100 for Alloy 617. Its advantage comes from having less chromium available for the salt to strip away in the first place.13Solar Energy Materials and Solar Cells. Corrosion behavior of alloys 600, 617, and hastelloy N in molten KCl salt In fluoride salts, the pattern holds: long-term tests of Hastelloy N and Hastelloy B3 in molten FLiNaK at 700 °C showed that corrosion rates were high initially as chromium and molybdenum dissolved from the surface, but slowed over hundreds of hours as the near-surface region became depleted, leaving behind a chromium-poor layer that corroded more slowly.14Journal of Nuclear Materials. Long-term corrosion behaviors of Hastelloy-N and Hastelloy-B3 in moisture-containing molten FLiNaK salt environments
Beyond alloy chemistry, there are mechanical engineering challenges. Pumps, valves, and shaft seals all have to function reliably at extreme temperatures in a chemically hostile liquid. In one experimental evaluation of a high-temperature shaft seal under MSR-like conditions, researchers observed that graphite bushing rings wore down primarily because of the thermal expansion of the stainless steel shaft. The initial contact generated friction and abrasion, but over time a small radial clearance formed as material wore away, reducing frictional heating and allowing the seal geometry to stabilize.15Progress in Nuclear Energy. Experimental evaluation of a high-temperature shaft seal under Molten Salt Reactor operating conditions It’s a useful reminder that making molten salt technology work at scale isn’t just about the salt itself; every component that touches the fluid needs to be rethought.
Grid-Scale Batteries With No Membranes
One of the more inventive uses of molten salt is as the electrolyte in liquid metal batteries designed for large-scale grid energy storage. A liquid metal battery consists of two liquid metal electrodes separated by a layer of molten salt. Because the three liquids have different densities and don’t mix, they naturally settle into three horizontal layers inside a single container, with no membrane or separator needed. That self-segregating structure makes the batteries easy to scale up, potentially long-lived, and cheap to manufacture.16Energy Storage Materials. A sodium liquid metal battery based on the multi-cationic electrolyte for grid energy storage
The concept was demonstrated in an early magnesium-antimony cell that operated at 700 °C using a molten chloride salt electrolyte. That cell was cycled at various current densities and achieved up to 69% round-trip energy efficiency.17PubMed. Magnesium-antimony liquid metal battery for stationary energy storage That efficiency isn’t as high as lithium-ion batteries, but liquid metal batteries are being designed for a different job: storing huge amounts of energy for hours at very low cost per kilowatt-hour, the kind of storage that could buffer an entire neighborhood or city district when the wind stops blowing or the sun goes down. The technology is still maturing, but the underlying physics are elegant: gravity and immiscibility do the work that expensive ion-exchange membranes do in other battery chemistries.
Turning Carbon Dioxide Into Useful Carbon
One of the more surprising uses of molten salt is as a medium for capturing and converting carbon dioxide. In the molten salt COâ‚‚ capture and electrochemical conversion process, COâ‚‚ dissolves readily into the hot salt thanks to high solubility, and an applied electrical current then splits it: solid carbon deposits on the cathode and oxygen gas releases at the anode.18Journal of Industrial and Engineering Chemistry. Recent advances in molten salt CO2 capture and electrochemical conversion to functional carbon materials The process doesn’t just sequester the carbon. It converts it into forms that could be valuable, including carbon nanotubes.
In chloride salt systems using calcium oxide as the COâ‚‚ absorber, researchers found that the composition of the salt strongly influences what kind of carbon comes out. In lithium-calcium chloride melts, the electrolysis produced mostly carbon nanotubes, with a carbon purity exceeding 99% and an overall electrolytic efficiency above 85%.19PubMed Central. The Effect of Molten Salt Composition on Carbon Structure: Preparation of High Value-Added Nano-Carbon Materials by Electrolysis of Carbon Dioxide Carbon nanotubes have applications in electronics, composites, and energy storage, so a process that converts a waste gas into a premium material while also producing oxygen is attracting attention from multiple directions.
The same electrochemistry has even been proposed for space exploration. Mars’s atmosphere is about 95% carbon dioxide, and a molten salt electrolysis system could process that atmosphere into solid carbon and breathable oxygen, two things that a crewed mission would need in abundance.20Device. Molten salt electrochemical CO2 conversion for producing oxygen and carbon on Mars The technology is still at the lab bench, but the concept illustrates how far the applications of molten salt have drifted from the industrial heat-treatment baths where they started.
Producing Hydrogen With Solar Heat
Another emerging application pairs molten salt with high-temperature electrolysis to produce hydrogen. In this scheme, a solar power tower heats molten salt in the usual way, but instead of running a steam turbine, the thermal energy is routed to a solid oxide electrolyzer operating at around 750 °C. The molten salt acts as a thermal buffer, ensuring the electrolyzer sees stable, consistent temperatures even when clouds pass over the solar field. A study modeling this coupling found that the solar tower technology working with molten salt as the heat-transfer fluid allows the high-temperature electrolyzer to operate at megawatt scale under steady conditions.21International Journal of Hydrogen Energy. Hydrogen production by coupling pressurized high temperature electrolyser with solar tower technology High-temperature electrolysis is more efficient than the low-temperature version used in conventional electrolyzers, so molten salt’s ability to deliver heat cheaply and steadily could lower the cost of green hydrogen.
What ties all these applications together is a basic physical insight: very few liquids can operate at extreme temperatures, carry large amounts of heat, remain stable for thousands of hours, and do it all without requiring high-pressure containment. Water turns to steam. Organic oils decompose. Liquid metals are expensive and hard to handle. Molten salts sit in a sweet spot that no other class of fluid occupies as well. Their weaknesses are real, particularly corrosion and the need for careful chemistry management over long service lifetimes, but the breadth of problems they solve keeps expanding as engineers find new ways to put hot liquid salt to work.