High measurement rate property measurement methods for molten salts
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Abstract
The demand for clean energy production and storage has inspired research in molten salt-based technologies. Understanding of how molten salts change with respect to temperature is vital to establishing efficient, cost-effective systems. Experimental measurements of physical properties and parallel x-ray studies of material structure can be used together to better understand how the molten salt speciation and behavior changes with temperature. Limited research into such materials means there are knowledge gaps and uncertainty in reported property values with respect to temperature due to difficulties in accurate experimental determination of properties. This is in part due to the stringent environmental requirements necessary due to the reactivity of the materials at elevated temperatures. Additionally, common high temperature techniques are time consuming, limiting the quantity of materials that can be evaluated. Many research efforts attempt to utilize MD modeling to fill these gaps and accelerate this process, however accurate physical measurements are still required to verify the results.
This thesis presents the development of glove box compatible techniques that are chosen with the intent of reducing measurement time while still providing accurate results of thermophysical properties as a function of temperature. Studies were performed on two candidate molten salts for one technology of interest, the Molten Salt Reactors (MSRs). The first is FLiNaK, the well-studied eutectic of the LiF-KF-NaF system with the composition of 46.5-11.5-42 mole percent. The second is a lesser studied salt NaF-ZrF4 with a composition of 53-47 mole percent. This salt was chosen as a surrogate for the NaF-ZrF4-UF4 fuel candidate used in the original MSR experiments between 1950 and 1970.
Properties measured in this work include viscosity, surface tension and density. Viscosity was measured with a falling ball viscometer adapted to high temperature applications. The intent of this system was to provide viscosity measurements as a function of temperature while minimizing sampling time and sample quantity. The applicable testing conditions were extended beyond the Stokes Flow regime (Reynolds Number < 1) typically used in the classic falling ball viscometer. A high temperature induction sensor was developed to measure the rate of the falling metallic ball, in this case Ti. Additionally, density and surface tension were measured simultaneously as a function of temperature with the maximum bubble pressure method. Combining these property measurements reduces total measurement time by eliminating the need for a second experiment. This method also addresses the effect of surface tension on density measurements that is commented on in published literature for molten salts, including FLiNaK and NaF-ZrF4. Considerations were made on both measurements to address environmental effects that might influence results. Characterization of salt samples before and after measurements were performed to provide context for the accuracy of the measurements with respect to temperature that may arise from these interactions with the environment and the materials used in the experiments.