// Thermal Analysis Labs July 27, 2020

Determining heats of mixing of molten salts with reaction calorimetry

In recent years, molten salts have become the focus of increasing research in nuclear and solar power applications as high temperature heat transfer fluids. Molten salts have unique advantages over other high temperature heat-transfer fluids, such as supercritical water: Lower vapor pressure decreases the risks associated with high pressure fluid, and fuels may be dissolved in the salt to create a system with built-in natural feedback loops that prevent thermal runaway of reactor cores – creating “inherently safe” reactor systems. In solar power systems, molten salts possess improved safety associated with a lack of hydrogen production and lower operating pressures as compared with supercritical water.

Figure 1. Specialized cells for measuring heats of mixing.

In the design of molten salt systems for different applications, it is important to thoroughly understand the chemistry of the system – including the heat of mixing to manage the heat from the initial production of the molten salt system. Specialty cells (Figure 1) can be used to conduct these measurements and enable better thermal understanding of these systems. In the cells, the two components are physically separated to the desired temperature, then heated past the melting point of both salts. The Ni separator is then removed, allowing the heat of melting to be observed.

Figure 2. Simulated and measured heats of mixing for a LiF4-ThF4 binary molten salt system.
Figure 3. SETARAM Multi HTC High temperature heat flux 3D DSC and drop calorimeter.

An example of the kind of data often generated from this system is seen in Figure 2. This data set, taken from a paper published by Dr. Ondrej Benes and his team at the Joint Research Center, shows the heats of mixing for a binary molten salt system.[1] The data was observed with a combination of drop calorimetry and 3D DSC techniques, measured using a SETARAM Multi HTC (Figure 3). The data serves to validate simulations of the expected heats of mixing of these systems and lend confidence to the thermal models that can be used to validate the principle and performance of intrinsically safe molten salt reactor systems.

Figure 4. Drop sensor and automatic sample charger for the drop calorimetry configuration of the Multi HTC high temperature calorimeter.

The SETARAM Multi HTC high temperature heat-flux calorimeter is the ideal system for measuring heat capacity and heat of mixing on high temperature molten salt systems: Its drop calorimetry mode enables unparalleled sensitivity in heat capacity measurement with a Calvet-style 3D DSC differential heat flow sensor that totally envelops the measurement chamber (Figure 4). For unparalleled precision in sample insertion, an automatic sample charger is available for the system. The reference chamber is located below the drop chamber to enable total thermal isolation and prevent interactions between the two chambers.

Figure 5. Measurement chamber of the 3D Heat flux DSC mode of the Multi HTC.

In heat flux 3D DSC mode of the Multi HTC, two ceramic chambers contain reference and sample crucibles (Figure 5). The differential heat flux DSC sensor uses 20 thermocouples that totally envelop the sample and reference chambers – enabling detection of over 90% of the available heat flow signal by conduction, convection, and radiation heat flow modes. This enables higher accuracy and sensitivity than the standard plate style DSC sensors which are only able to detect conduction into the heat flow sensor.

[1] E.Capelli, O.Beneš, M.Beilmann, R.J.M.Konings. The Journal of Chemical Thermodynamics. 2013. 58. 110-116. DOI: 10.1016/j.jct.2012.10.013


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