Thermophysical Structure of High-Temperature Molten Salts Using Spectroscopic and Other Methods

Compared to conventional nuclear reactors, molten salt reactors (MSRs) operate at higher temperatures, which allow them to achieve better thermal efficiency, and at lower pressures, which can enhance safety. MSRs can be useful for generating electric power, providing process heat, or waste incineration of radioactive actinides. An accurate database on thermophysical-chemical and transport properties of the molten salts such as density, specific heat, solubility, diffusivity, viscosity and thermal conductivity, is critical to the design of MSRs. However, only a limited data set is currently available on molten salts in different environments due to the difficulties involved in measuring these properties. A recent experiment using optical transient grating spectroscopy (TGS) measured thermal conductivity data with a behavior that is consistent with theoretical expectations, unlike previous measurement techniques, which varied widely in their results and exhibited the wrong behavior. We propose to develop TGS into an instrument to measure simultaneously the speed of sound and thermal diffusivity in molten salts at reactor temperatures. Such measurements, in combination with measurements of density and specific heat from conventional methods, may be useful in deriving the needed thermophysical properties of molten salts, such as FLiBe, used in reactors. The proposed system will support multiple thermophysical property measurements using a set of apparatuses, including TGS, differential scanning calorimetry, and a density apparatus, for FLiNaK salt, a surrogate to FLiBe, at various temperatures. The developed TGS technology, together with the various conventional apparatuses, will provide an inexpensive and compact measurement solution suitable for harsh industrial environments for real-time monitoring in a molten salt reactor bleed system during its operation. Such a critical dataset will be useful for understanding the salt behavior in MSRs and reactor operations and further help with the design and modeling of heat exchangers and other MSR components.