## 2 Molten Salt Mixtures
### Molten Salt Mixtures as Heat Transfer Fluid and Storage Medium  
MSs are predominantly utilized for the storage of heat, presenting a sustainable method for capturing and storing thermal energy. Their main application is found in CSP plants and nuclear facilities [15], where their capacity to store vast amounts of heat at elevated temperatures facilitates the generation of electricity and/or heat. Nitrates and nitrites are among the most commonly used HTFs in current applications. However, the production of these salts is geographically and quantitatively limited, with the majority of global reserves located in Chile and Peru [15]. In nuclear facilities, an instance of application is Nuclear Energy Systems (NESs), comprising two primary components: an MS loop that transfers heat through a secondary coolant from the nuclear reactor to a storage tank, and a power cycle loop, which includes various power cycles converging at the point of common coupling. These cycles might use Rankine, Brayton, or combined Air-Brayton configurations, utilizing different working fluids to generate power. A variant of an NES is its hybrid version, which combines various energy generation systems to enhance efficiency. This includes incorporating nuclear reactors, renewable energy sources, process heat applications, and different energy storage solutions [16]. Regarding CSPs, they transfer thermal energy by collecting solar radiation using an array of mirrors or heliostats to concentrate it onto a receiver, which then transfers the heat, directly or indirectly, to the MS. Technologies such as PTC, solar power towers, and linear Fresnel reflectors are instrumental in this process. Each varies in its method of concentrating solar energy but is unified in its objective to maximize the capture of thermal energy and its transfer to the MS.  
A prime example of MS use in CSP systems is \"solar salt\", a binary mixture of 60% NaNO\({}_{3}\) and 40% KNO\({}_{3}\) by weight. Its ability to remain thermally stable in a liquid state at temperatures of up to 600 \({}^{\circ}\)C enables efficient heat transfer and energy storage,exemplified in the Solar Two central receiver system in California and numerous solar thermal plants in Spain [17]. The first demonstrations in Europe of MS deployment in linear CSP configurations include projects like the experimental facility PCS of the ENEA Casaccia Research Centre (Italy) and the direct MS loop located at Priolo Gargallo (Sicily, Italy), addressing the integration challenges in CSP technology [18]. For the storage of MSs, sophisticated containment systems are employed, designed to minimize heat loss and ensure the longevity and safety of the stored thermal energy. These systems typically involve insulated storage tanks that can maintain the high temperatures of MSs without significant thermal degradation over time. The storage tanks are engineered from materials specifically chosen for their resistance to the corrosive properties of MSs and their ability to endure thermal stresses arising from temperature fluctuations [15]. There are two primary types of MS thermal storage options utilized in CSP plants: the thermocine system and the two-tank system. The two-tank system operates on a principle where a hot tank and a cold tank separately store the salt, akin to conventional storage schemes in CSP plants. Conversely, the thermocine system employs a single tank that houses both cold and hot salt layers. In a thermocine tank, the salt layers are divided by a thermocine--a sharply defined layer where the temperature changes dramatically. This layer acts as a barrier, preventing the hot and cold salts from mixing, thereby maintaining distinct thermal zones within a single storage vessel. When charging, the cold salt moves from the tank's colder region, passes through a heat exchanger, and enters the warmer section, effectively storing thermal energy. During discharge, the process reverses, with the hot salt flowing out through the heat exchanger back to the colder part of the tank.