Molten salt for advanced energy applications: A review

The primary uses of molten salt in energy technologies are in power production and energy storage. Salts remain a single-phase liquid even at very high temperatures and atmospheric pressure, which makes molten salt well-suited to advanced energy technologies, such as molten salt reactors, or hybrid energy systems. The molten salt cooled reactor is an advanced nuclear reactor concept that utilizes molten salt as either a coolant for solid fuel or as a fuel salt. The liquid phase provides orders of magnitude higher heat capacity per cubic meter than the gas phase. This, coupled with the low-pressure environment required to maintain the liquid phase, provides significant advantages in terms of compact-sized systems constructed with relatively thin walls. The heat from a heat-generating process is transferred to a heat transfer media and can be extracted later using a secondary power cycle. There are several types of facilities that use thermal energy storage with molten salts, such as concentrated solar power plants (CSP plants) or nuclear hybrid energy systems (NHES). A CSP plant is a power production facility that uses a broad array of reflectors or lenses to concentrate solar energy onto a small receiver. Since molten salt remains in the liquid phase, it has excellent heat retention properties, meaning heat from a solar-generation process can be stored for an extended period for later use. A Nuclear Hybrid Energy System (NHES) refers to several energy systems combined to generate energy more efficiently, such as nuclear reactors, renewable energy sources, process heat applications, and energy storage. The selection of a salt type for a nuclear reactor or a thermal storage system requires careful consideration of the chemical and thermodynamic properties of the candidate salts. Different energy technologies will require different salt types, based on temperature and fluid property requirements. Fluoride salts are often the primary candidate salts for nuclear reactor systems. Chloride salts are another category of candidate salt that have been considered for power production because chloride salts often exhibit similar behavior to fluoride salts. Nitrate-nitrite salts contain NO3 and NO2 and are used in solar applications. As with other nuclear reactors, molten salt systems involve radiological and chemistry challenges, including tritium production and corrosion. Tritium production can be problematic in a reactor system because it can be a hazard to human operators. Tritium is difficult to contain; therefore, the production of tritium must be minimized. Corrosion of structural materials is also an area requiring further study in molten salt systems. Corrosion in a molten salt system differs from standard nuclear reactor systems due to the lack of a passive oxide film on the surface of structural materials, making it necessary to mitigate corrosion by either purifying the salt, controlling its redox potential in a reducing state, or using redox buffers. Additionally, since molten salt reactors are constructed with much thinner structural members due to low-pressure loads, reducing reactor operating lifetime and/or marginally increasing the structural thickness to provide additional corrosion allowance may be acceptable design approaches. The behavior of volatile fission products in Fluoride and Chloride salts is also a consideration due to the volatility of insoluble fission products that precipitate and plate out on surfaces affecting thermal hydraulic parameters. Fission products such as cesium (Cs), iodine (I), strontium (Sr) and other salt seekers, have a complexing nature with fluoride (F) and iodine (I) and their behavior in molten salt reactors will have to be addressed. Molten salt reactors present a particular challenge for recycling fission products including solubility, volatility, and precipitation behavior, and how the fission products change the corrosivity of the salt melt. Current nuclear fuel recycling technology will not accommodate molten salt streams and will need to be redesigned. New molten salt recycling designs and chemistry will ultimately need to be applied to a variety of fluoride and chloride salts mixtures. All thermal energy facilities either utilize the heat energy produced directly, such as space or process heating, or convert a portion of the heat energy to some other energy form such as electricity. The energy conversion cycles utilized in most (if not all) other types of base-load power plants (e.g., coal-fired power plants, gas-fired power plants, and nuclear power plants) are the Rankine and Brayton power conversion cycles. MSRs are also coupled with both Rankine and Brayton power-conversion cycles to transform heat into other energy forms. The feasibility for cycle coupling with MSRs depends on a variety of factors; two of the more important factors are technology readiness level (TRL) and efficiency. The application of existing steam cycle designs will likely require modifications to equipment. Steam generators and reheaters will present a particular problem in accommodating the molten salt. The adaptation of the plant to the molten salt reactor will require trade studies to obtain information necessary for further design.