MOLTEN SALTS AS POTENTIAL HEAT TRANSFER COOLANT

    The high temperature required for the process application (such as Hydrogen Production) and the required distance between facilities due to licensing restrictions restricts the number of heat transfer fluids that can be used to couple a nuclear plant with a process plant. In selecting a heat transfer fluid, it is desired to have a fluid that is stable at the maximum possible outlet temperature of the reactor. This restricts the heat transfer fluid to simple fluids, such as noble gases, simple alkali, and alkaline-earth halide salts. Of the noble gases, helium is one of the potential heat transfer fluids. Of the halide salts, fluoride salts and chloride salts are both stable options over the proposed operating temperature range for NGNP (700–900°C). An additional consideration in choosing the one of these fluids is the pumping power required for the fluid.
    Following are the characteristics that make the liquid salts potential candidates for process heat transfer applications (Sabharwall, et al. 2004):
    1. High Boiling Point 2. Low Vapor Pressure3. Large Specific Heat and Thermal Conductivity 4.High Density at Low Pressures.
   The volumetric heat capacity is an important parameter in determining the amount of energy that can be contained in a unit volume of salt. The larger the volumetric heat capacity, the less salt is needed to transport the heat from the reactor to the chemical plant, which in turn provides the practical benefit of reduction in the size of the pumping equipment (Ambrosek, 2010).
    The melting temperature of a salt mixture is the most important criteria in the selection of a salt. In the NGNP application, it is desired to get the melting temperature of the salt mixture to as low as possible, while still maintaining thermal stability of the salt at NGNP high operating temperatures. One way to lower the melting point of a salt is to combine multiple salts to form a salt of eutectic composition. These eutectic compositions have a much lower melting temperature than the individual salt components and are characterized by a single melting point. For example, for the FLiNaK salt, the individual constituents LiF, NaF, and KF have a melting point of 830, 880, and 912°C, respectively. When the three components are combined to form a eutectic composition, FLiNaK, the melting point of the salt drops to 454°C. These deep-well eutectic compositions are commonly formed in mixtures of salt components (Ambrosek, 2010). However, most ternary and higher order systems of salt mixtures do not have much experimental data available for the density, viscosity, thermal conductivity, etc.
    The physical properties of molten salts in the liquid phase are similar to the properties of water at room temperature. Molten salts possess high volumetric heat capacities, CP. In addition, they have low vapor pressures (<5 Pa @ 900°C), which in turn reduces the stress requirements in the piping. The thermal conductivity of molten salts is also quite high, in the range of approximately 0.4 to 1 W cm-1 K-1 bracketing the value of water at room temperature 0.6 W cm-1 K-1 (Ambrosek, 2010).
    The use of salts as heat transfer fluid, instead of He, improves the efficiency of the whole reactor system, electric and hydrogen production, by up to 0.6% (Davis, 2005). This is if 50 MW of the 600 MWth is transported by use of FLiNaK salt to the hydrogen chemical plant. For comparison, it requires a 100°C temperature increase in the reactor outlet temperature to increase the efficiency of the whole plant, electrical and chemical, by 1.1% (Ambrosek, 2010). This increase in efficiency from the use of salts increases NGNP’s economic viability.
    Much of the recent work in molten salt research has been performed by Anderson, Sridharan, and coworkers at the University of Wisconsin. Their research program has been multi-faceted and includes the following areas of work:
    1. Static, isothermal screening of corrosion of a variety of alloys in FLiNaK and KCl-MgCl2
    2.Construction of loop systems for evaluating heat transfer issues
    3.Thermal modeling
    4.Modeling of thermo-physical properties of salt compostions.
    The research performed at the University of Wisconsin, along with the future directions identified as a result of this research, is summarized in the following sections.