First-of-its-kind system tests reactor elements in continuous loop.

At a research facility best known for pioneering nuclear innovation, scientists at Idaho National Laboratory have built a first-of-its-kind test system that moves molten salt through a loop of steel under intense heat – hoping to understand how advanced materials and sensors perform within the harsh conditions of nuclear reactors.

As advanced reactor designs push toward higher temperatures and more efficient performance, the materials and instruments used to operate and measure their condition face increasing demands – particularly in systems that rely on salt as coolant or fuel.

Before these reactors can be brought online, researchers must prove that critical components can withstand years of exposure to environments far more corrosive and volatile than those found in traditional nuclear plants.

Rather than using solid fuel rods and pressurized water, molten salt reactors circulate liquid salt mixtures – sometimes carrying dissolved nuclear fuel – at temperatures that can exceed 700 degrees Celsius (1,292 degrees Fahrenheit).

This approach allows for passive safety features, more efficient heat transfer, and the potential to supply both electricity and industrial-scale process heat.

But the same chemistry that makes molten salt reactors promising also makes them punishing: high heat, moving fluids, and reactive elements combine to create a uniquely aggressive cocktail for reactor components.

In response to that volatility, researchers at Idaho National Laboratory have built a one-of-a-kind closed-loop system that circulates molten salt through stainless steel piping at sustained high temperatures – recreating the corrosive, high-flow conditions reactor components are expected to endure.

Designed to run continuously over long durations, the test loop makes it possible to observe how materials, sensors, and flow mechanics behave under extended exposure to heat and stress.

"The instrumentation and sensor testing in flow loop environment is one-of-a-kind," said Ruchi Gakhar, a lead scientist for INL's Advanced Technology of Molten Salts program. "By understanding how sensors react to high temperature flowing molten salt, we hope to advance the readiness of future molten salt reactors."

Outfitted with access points for instruments and sensors, the system allows researchers to track how the salt behaves as it moves – how it holds or transfers heat, how it interacts with different materials, and how long those materials last under stress.

Unlike earlier test setups that required shutdowns and disassembly to gather results, this loop can run continuously, providing steady feedback without breaking operation.

"Most test loops focus on testing the structural materials," said Gakhar. "After a few hours of operation, they dismantle the loop to study how the materials have degraded."

When not in use, the system can drain into a dedicated holding tank – extending the lifespan of the equipment by preventing residual corrosion and making it easier to restart tests without rebuilding or replacing key components.

"Our loop at INL is unique because it serves as a test bed for advanced electrochemical sensors and bubbler instruments," Gakhar continued. "These instruments allow us to monitor and investigate material performance in real-time while the loop is still operational. This approach has not been implemented or explored in flow loops at other institutions."

Constructed from stainless steel, the molten salt flow loop functions similarly to that of a car radiator, which circulates coolant through the engine to prevent overheating and maintain optimal temperature.

The liquid within the loop contains a special mixture of lithium chloride and potassium chloride salts; to keep the solution clear from contamination, an inert gas is used to fill the remaining space in the tube.

Equipped with five electrode ports for electrochemical experiments; bubbler dip-tube ports to measure fluid density, surface tension, and salt levels; and special devices to monitor temperature changes throughout the system, this system has been meticulously designed to allow the scientists to have as complete a picture of the heat transfer properties of molten salts.

Data collected will inform the design and operation of the Molten Chloride Reactor Experiment, a fast-spectrum molten salt system under development at INL in collaboration with Southern Company and TerraPower.

Backed by the Department of Energy's Advanced Reactor Demonstration Program, the project aims to test the viability of high-temperature salt reactors as a new class of reliable, low-carbon energy systems.

As part of the collaboration, INL will handle the synthesis and management of the fuel salt, operate the reactor, and oversee post-operation deactivation and disassembly.

The insights gained from the molten salt flow loop will be essential for enhancing the durability and performance of these reactors, ultimately helping to reduce operational costs and minimize maintenance demands.

"This will be a big step toward building molten salt reactors that last longer and require less maintenance, reducing costs and improving reliability," said John Carter, manager of the Advanced Technology of Molten Salts department at INL. "This work will accelerate us toward our advanced energy future. It will enable development of advanced corrosion-resistant materials, sensors, and instrumentation for ultra-high-temperature applications."