US scientists have collected rare new data on molten reactor salts that could help accelerate the development of safer, more efficient, and easier-to-license future nuclear reactors.

Researchers at Oak Ridge National Laboratory (ORNL) in Tennessee demonstrated advanced experimental methods to measure how uranium-bearing molten salts transfer heat and flow inside molten salt reactors (MSRs).

The work could improve the design and modeling of molten salt reactors, which are one of the leading concepts for advanced nuclear energy systems. According to the team, the measurements focused on two crucial properties required for the design and modeling of these reactors: thermal conductivity and viscosity.

Still, reliable experimental data for both properties remained scarce for years. This has limited efforts to optimize reactor designs and support regulatory approval pathways. To address the issue, the ORNL team tested several candidate molten salt mixtures using specialized tools that they developed at the laboratory.

Rare nuclear measurements

Molten salt reactors are nuclear fission reactors in which either the fuel and/or the coolant is a molten salt. Meanwhile, molten salts liquify at elevated temperatures and can store massive amounts of thermal energy at atmospheric pressure. Once utilized as fuel, the molten salt is dissolved with fissile material, like uranium-235, plutonium-239 or uranium-233.

For the project, the ORNL researchers used two specialized instruments to study how these molten salts behave under reactor-like conditions. One system, known as the variable gap system, measures thermal conductivity by tracking how heat moves through molten salt samples.

The second tool, a rolling ball viscometer, determines viscosity by measuring how quickly a small ball travels through the liquid salt. “Some of the collected data are completely unique and fill gaps necessary to MSR development,” Anthony Birri, PhD, an R&D associate at ORNL, stated.

Together, the measurements provided key insight into how molten salts circulate, transfer heat, and perform inside future reactor systems. Even small variations in these properties can significantly affect reactor efficiency, cooling performance, and operational safety.

New tools for MSRs

According to Birri, the new information will help expand the Molten Salt Thermal Properties Database. This is a growing resource used to support advanced reactor modeling, optimization, and future licensing efforts.

“We’re collaborating across the national labs to further expand the Molten Salt Thermal Properties Database, a vital resource for supporting the licensing and optimization of advanced nuclear technologies,” Birri noted in a press release.

The novel technology has attracted growing global attention as governments and private companies push to develop advanced nuclear systems that are capable of delivering carbon-free electricity alongside renewable energy sources.

ORNL, which reportedly pioneered molten salt reactor experiments in the 1960s, is one of the leading research centers for the technology. The latest effort was also supported by the Department of Energy’s (DOE) Office of Nuclear Energy Molten Salt Reactor Program.

“Jacob Yingling and Bill Phillips (Idaho National Laboratory), Bruce McNamara (Pacific Northwest National Laboratory), and  Amanda Leong and Jinsuo Zhang (Virginia Polytechnic Institute and State University) contributed to this effort,” ORNL pointed out.