Researchers at the Department of Energy's Oak Ridge National Laboratory are helping to enable the next generation of abundant, affordable nuclear energy by combining 80 years of know-how with the latest scientific techniques, facilities and equipment. The lab's longstanding expertise in degradation of materials in the harsh environments of nuclear reactors make it the go-to place for a resurgence of interest in liquid metals and molten salts for both advanced fission and fusion reactors.

"Fission and fusion are critical technologies that are of great interest to researchers and industry," said Rishi Pillai, who leads ORNL's Corrosion Science and Technology Group. "A number of these new reactor concepts that are being pursued by industry use molten salts and liquid metals, which have been historical areas of expertise for ORNL."

Multiple companies are looking at molten salt and liquid metals for fusion and advanced fission because they are excellent heat-transfer fluids with high heat capacity that promise high operating temperatures at lower pressures, leading to higher efficiency. However, molten salt and liquid metals create harsh environments that can degrade the materials of the reactor, making corrosion research a critical area of inquiry to enable these concepts.

While many think of rust on a metal surface when they envision corrosion, in nuclear reactors the degradation of materials is much more complex. High-temperature liquid metals and molten salts create challenges for structural materials. Key alloying constituents can become depleted. Materials can embrittle, crack and fail. Strengthening phases can dissolve and detrimental phases can develop. Mechanical stress and neutron irradiation can further exacerbate these effects.

Corrosion capability

ORNL's corrosion program looks at how a variety of materials, such as metal alloys, degrade over time in the presence of corrosive elements at high temperatures while being battered by high-energy neutrons released by nuclear processes. Researchers in Pillai's group are busy equipping a state-of-the-art laboratory at ORNL's new Translational Research Capability building, consolidating research that was spread across more than a dozen lab spaces at ORNL into a compact, collaboration-friendly space. The team also taps into the unique fusion and fission expertise across the lab and one-of-a-kind facilities such as ORNL's High Flux Isotope Reactor, which is the most powerful reactor-based steady-state source of neutrons in the U.S.

ORNL developed expertise in molten salts with the construction of the Molten Salt Reactor Experiment (MSRE) in the 1960s. Although MSRE operated for less than five years as a proof-of-concept, it showed that molten salts could function as both a fuel carrier and coolant for a reactor while enhancing fission safety and efficiency. ORNL was also involved in corrosion research for the Experimental Breeder Reactor II, which was cooled by liquid metal, in this case sodium. EBR II operated for nearly 30 years at what is now known as Idaho National Laboratory. EBR II shut down in 1994.

"The leaders of this group always focused on ensuring they passed their knowledge on to successors and maintained that scientific expertise," said Marie Romedenne, a globally recognized researcher in high-temperature corrosion, who leads ORNL's liquid metal fusion and advanced fission program. "They had the foresight to see that this technology would be important, so they maintained the facilities and the technical knowledge we needed to be ready when the moment was right to push this toward deployment."

While liquid metals and molten salts have long been of interest in fission energy, they have only recently taken on heightened importance for fusion energy. The predominant architectures for magnetic confinement fusion - donut-shaped tokamaks and cruller-shaped stellarators - both create extremely high-temperature plasmas that can initiate the fusion of hydrogen isotopes to make neutrons and helium, releasing kinetic energy.

One of the challenges facing companies building fusion devices is how to convert that energy to electricity while cooling the reactor and even creating fuel on site. These tasks are accomplished by a blanket that surrounds the plasma, absorbing heat and energy from neutrons exiting the plasma.

"Research has been focused for decades on how to create and sustain fusion in the plasma, but the blanket is also a formidable challenge due to its multi-purpose nature and the complex material interactions that take place there," Pillai said. "Incorporating molten salts into the blanket is a concept that needs more experimental investigation to explore its potential for resolving this challenge."

ORNL researchers are working with colleagues at other national labs and several private companies to design blanket and fuel cycle systems. Pillai's group is looking at a variety of alloys, both conventionally and additively manufactured, along with coating systems that could withstand liquid metals or molten salts over extended periods of time in the harsh fusion environment.

ORNL's Corrosion Science and Technology Group specializes in studying individual and combined effects of corrosion stress and irradiation. Researchers expose a variety of materials to these types of stressors and use cutting-edge characterization techniques to understand what's happening at the atomic level. Scientists work with some of the most difficult materials, aided by the deep expertise of technicians, insights from models run on high-performance computers and access to facilities specifically designed for these materials.

"Our research blends state-of-the-art experimental observations of real-world conditions with high-fidelity modeling of material degradation, providing rapid engineering data on material performance while enabling deep mechanistic understanding of material-environment interactions," said Pillai. "This unique combination of competencies makes ORNL the leading lab to tackle the fission and fusion challenges."

UT-Battelle manages ORNL for the Department of Energy's Office of Science, the single largest supporter of basic research in the physical sciences in the United States. The Office of Science is working to address some of the most pressing challenges of our time. For more information, please visit energy.gov/science. - Greg Cunningham