ORNL completes challenging molten salt property measurements

Oak Ridge National Laboratory announced the completion of a set of experiments measuring the viscosity and thermal conductivity of several uranium-bearing molten salts, filling in gaps that could help with the development of molten salt reactors.

"Because fuel qualification depends on understanding how liquid fuel salts behave under reactor conditions, precise and well-characterized thermal property data is essential for MSR design and safety analysis," said Tony Birri, an ORNL research and development staff member.

According to Birri, the viscosity and thermal conductivity of uranium-bearing salts are "notoriously challenging" to measure with a high degree of accuracy because of the high temperature conditions and required materials for salt containment.

"Thermal conductivity is measured by using samples which are fractions of a millimeter thick in order to avoid the confounding effects of radiative heat transfer," said Birri. "This involves the design of custom systems with very tight tolerances and ability to finely tune the gap size."

Birri and his team use a specialized variable-gap apparatus to measure thermal conductivity, calibrating the system over a wide range of thermal conditions to develop correction factors to account for heating losses.

Barriers to viscosity measurements include corrosion in the system and bubbles in the sample, so the team used highly purified samples and vibration tools to drive out bubbles. They also ensured good compatibility with the salt-wetted materials.

The team has measured these properties for molten salt mixtures of sodium fluoride and uranium tetrafluoride (NaF-UF₄); sodium fluoride, potassium fluoride, and uranium tetrafluoride (NaF-KF-UF₄); and sodium chloride and uranium trichloride (NaCl-UCl₃). The results are published in ORNL's Molten Salt Database.

"Tools like the MSD, informed by ORNL's measurement capabilities, aim to improve the reliability of simulation tools and ultimately accelerate the deployment of future molten salt reactors," he said. The team plans to continue studying salt systems bearing UF₄ and UCl₃.

"The domestic MSR developers are considering both UF₄- and UCl₃-bearing compositions for their fueled salts, although the exact compositions of the carrier salts may differ from what we have studied into the laboratory," said Birri.

A molten salt turning point: Both Kairos Power and TerraPower plan to make use of molten salts for the reactor projects they broke ground on in April-though not as fuel-bearing salts.

Kairos is constructing Hermes 2, a 50-MWe fluoride salt-cooled high-temperature reactor. The company expects to begin operations by 2030.

TerraPower is constructing Kemmerer-1, a 345-MWe sodium-cooled fast reactor that includes an integrated molten salt-based heat storage system. Construction is expected to be completed by 2030.