FY2022 Progress Report on Viscosity and Thermal Conductivity Measurements of Molten Salts

Abstract

This report describes the development of and experimental measurements made with the thermal conductivity and viscosity measurement systems for molten salts at Oak Ridge National Laboratory. The thermal conductivity system is based on a steady-state measurement technique in which a heat flux is driven across a variable gap, and the changes in temperature difference across the gap are measured as the gap size is varied. The system comprises an inner containment, which possesses the heating element, and an outer containment, which possesses the salt specimen and cooling channels. The gap is formed between the outer bottom of the inner containment and the inner bottom of the outer containment. The viscosity system is based on the falling-ball technique in which a salt-filled tubular crucible is fixed at an angle and a ball rolls through the salt. The terminal velocity of the ball can be converted into viscosity via a calibration factor calculated for crucible-ball combinations which account for thermal expansion and flow characteristics. Modifications were made to both systems this fiscal year. Modifications to the thermal conductivity system included adding guard heaters to prevent axial heating losses, and modifications to the viscosity system included changing the crucible design to prevent air pocket formation and providing the ability to conduct measurements with a glass or steel crucible.

FY23 Progress Report on Viscosity and Thermal Conductivity Measurements of Molten Salts

“…Temperatures were measured for each data point over five gaps ranging from 0.35 mm to 0.05 mm. The results of four temperature setpoints (680, 716, 760, and 800 • C) are shown in Figure 10 comparing data from the previous FY [8], experimental literature data from Smirnov [44], kinetic theory based on a model derived by Gheribi [21], and an ab inito model [21].…”

“…As opposed to designs in previous reports [8,16], a simplified, two-stage crucible design was designed and fabricated in this work, as shown in Figure 1. The crucible consists of a loading section (i.e., the first stage) and a measurement section (i.e., the second stage).…”

“…Previously fabricated designs involved a third stage, which was a second holding section; the intent was to be able to tilt the crucible back and forth, conducting multiple measurements with the same ball. However, gas pocket formation was an issue with this design implementation [8]. The simplified design considered herein remains in a static orientation, and new balls are introduced to the crucible for each measurement.…”

“…ORNL recognizes that characterization of heat capacity is generally lacking for many key mixtures as well, but other DOE-NE funded laboratory and university partners are focusing more heavily on measurements of that property specifically [15,14,23,28,42,50]. The molten salt mixtures measured herein-and which have been measured in previous reports on this work [8,16]-are all alkali halide mixtures, which represent either coolant salts or subsystems to higher-order actinide-bearing salts that may be considered as MSR fuel. However, the measurement systems developed and reported on herein have been designed to enable direct measurements of actinide-bearing mixtures, which will be crucial for providing accurate and validated thermophysical property data to MSR thermal models.…”

Survey of Relevant Data from the MSRP to Guide Development of MSR Chemistry Modeling Benchmarks

“…Under the NEAMS Structural Materials & Chemistry technical area, development has been ongoing for the Molten Salt Thermal Properties Database -Thermophysical (MSTDB-TP) and currently includes temperature-dependent functions for four property types across a collection of fluoride and chloride salt mixtures, including density functions for 212 mixtures, viscosity functions for 127 mixtures, heat capacity functions for 45 mixtures, and thermal conductivity functions for 25 mixtures [63]. Future work includes adding data for new mixtures as it becomes available under QA practices as well as the potential inclusion of new other properties to the database such as surface tension and others.…”