Molten salt is a substance that is solid at room temperature but becomes a liquid when heated, without being dissolved in water. This high-temperature melt possesses unique thermodynamic properties. The concept of using melted salts is not new, but their application has seen a major resurgence in modern energy technology to improve the efficiency and safety of power generation systems.
Defining Molten Salt: Composition and Phase Change
Molten salts are fundamentally ionic compounds, meaning they are composed of positively and negatively charged ions. Unlike common table salt dissolved in water, which forms an electrolyte solution, a molten salt is the liquefied form of the compound itself. These substances typically require temperatures well above 200°C to achieve a liquid state, depending on their specific chemical makeup.
The compounds used often include mixtures of nitrates, chlorides, or fluorides because of their thermal stability and manageable melting points. Engineers frequently employ a eutectic mixture, which is a blend of two or more salts at a specific ratio. This combination yields the lowest possible melting temperature for that system, considerably lower than the melting point of any single component salt. For instance, a common mixture of sodium and potassium nitrate, called solar salt, has a melting point near 220°C. This lower freezing point is important for preventing the salt from solidifying within pipes and equipment.
Essential Thermal and Physical Properties
The characteristics of molten salt make it uniquely suited for high-temperature applications. A primary property is its high heat capacity, which is its ability to absorb and store a large amount of thermal energy per unit of mass. This capacity allows molten salt to act as an effective thermal energy storage medium, holding heat for extended periods.
Another significant advantage is the low vapor pressure of molten salts, meaning they do not easily boil even at extremely high operating temperatures. This allows systems to run safely at or near atmospheric pressure, eliminating the need for high-pressure containment vessels found in many conventional power plants. Molten salts also exhibit high thermal stability, remaining chemically sound when heated to temperatures between 500°C and 800°C. Their effective heat transfer, combined with this stability and low pressure, makes them ideal candidates for next-generation energy systems.
Current Uses in Industry and Energy Storage
The most established commercial application of molten salt is in Concentrated Solar Power (CSP) plants, where it is used for thermal energy storage (TES). In a CSP tower system, mirrors focus sunlight onto a receiver at the top of a central tower. Molten salt is pumped up the tower, absorbs the concentrated solar energy, and is heated to temperatures around 565°C.
This superheated salt is circulated to a hot storage tank, acting as a thermal battery that retains the heat with minimal loss. When electricity is needed, the hot salt runs through a heat exchanger to boil water and create steam, which drives a conventional turbine. This allows CSP plants to provide “dispatchable” solar power, meaning they can generate electricity on demand, overcoming the intermittency of direct sunlight. Beyond solar, molten salts are also used in various industrial processes, such as specialized chemical synthesis, aluminum production, and as quenching baths for heat treating metals.
The Function of Molten Salt in Nuclear Reactors
Molten salt technology is central to a class of advanced nuclear designs known as Molten Salt Reactors (MSRs). In these reactors, the salt can serve two primary functions: as a coolant or as a solvent for the nuclear fuel itself. In liquid-fueled MSR designs, the fissile material, such as uranium or thorium, is dissolved directly into the molten salt mixture, creating a liquid fuel that also functions as the primary coolant.
This unique configuration allows the reactor to operate at high temperatures, up to 750°C, which significantly increases the thermal efficiency of the power conversion process. MSRs are designed with inherent, passive safety mechanisms that rely on the salt’s physical properties. If the reactor begins to overheat, the liquid salt naturally expands, pushing the fuel atoms farther apart and automatically slowing the nuclear fission reaction.
Many MSR designs incorporate a freeze plug at the base of the reactor, which is actively cooled to remain solid. In the event of a power loss or severe overheating, this plug melts, allowing the entire liquid fuel charge to passively drain into a separate, subcritical holding tank where it cools and solidifies. This permanently stops the nuclear reaction. Alternatively, some MSR concepts use molten salt solely as a low-pressure coolant to transfer heat from solid fuel elements, benefiting from the salt’s high thermal stability and low-pressure operation.