Core concern

Samuel Briggs and Julie Tucker checking the leads on a device.

Samuel Briggs and Julie Tucker check the leads to a device that will check for minute cracks in materials to be used in nuclear reactors.

Renewed interest in nuclear power as a viable option for generating electricity has been accompanied by steady progress in reactor design. Advanced reactors offer the promise of greater fuel efficiency and less radioactive waste generation compared with the water-cooled models that have dominated the nuclear power landscape for decades. Newer designs, however, will operate at higher temperatures and use highly corrosive coolants — like liquid metal, molten salt, or high-temperature gas — all of which would rapidly degrade many of the materials used in conventional nuclear reactors.

“The challenge for engineers is to develop new materials that can survive these exceptionally harsh conditions for the lifetime of a nuclear facility,” said Samuel Briggs, assistant professor of nuclear engineering at Oregon State University.

To that end, the Department of Energy’s Idaho National Laboratory is leading the research and design of the Versatile Test Reactor. Its primary purpose will be testing new materials and technology for nuclear reactors. The proposed VTR could produce neutron damage in test samples 20 times faster than can be created by current water-cooled test reactors. That would allow researchers to observe the effects of years — even decades — of radiation-induced damage after just months of testing. Plans call for liquid sodium as its primary coolant, but the VTR would be configured to accommodate experiments that re-create a variety of operating environments.

Preparatory research is already underway to develop instrumentation and tools for conducting and monitoring VTR experiments. For one such project, Briggs has teamed up with Julie Tucker, an associate professor of materials science at Oregon State. The goals of their work, which is funded by the DOE, are to develop a system to measure the degradation of test materials in situ, while they are exposed to radiation within the VTR core, and to transmit that data to researchers.

Specifically, the team set out to measure structural degradation in small sections of 3/16-inch-thick stainless steel rods under mechanical stress. A scissors-like actuator repeatedly applies gentle tensile and compressive loads to the sample — movement that replicates the physical strain experienced by reactor components.

“Over time, small defects propagate. Eventually, the metal will crack, even though the stress applied each time is small,” Tucker said. “We’ll measure the growth rate of the crack. That’s important, because engineers want to know the safety margins for materials they use to build reactors: How long does it take it to fail once a crack is detected in a component?”

Tucker and Briggs are testing several diagnostic techniques to gauge sample damage, such as acoustic emission monitoring. When cracks propagate, they create faint audible signals. “By amplifying and characterizing the wave forms, we can ascertain information about how fast the material is deteriorating,” Briggs explained.

They’ve also considered using established diagnostic techniques that measure changes in electrical resistance, which occur in a conductive material as cracks develop. But because the coolants themselves are highly conductive, such techniques are likely to produce inaccurate readings, according to Tucker.

Characterizing the sample’s structural integrity is not even the trickiest part.

“The biggest problem is how to take the data about the minute cracks that we detect and transmit it outside a nuclear reactor,” Tucker said. “This project is proof of principle that we can do that.”

In previous work, done separately, Tucker and Briggs exposed materials to radiation, corrosive coolants, and mechanical strain, then characterized their stability as well as any degradation at microscopic and atomic levels. But in this case, they’ll need to collect data in real time during ongoing reactor operations.

So far, the researchers have tested their system in gases, including air and supercritical carbon dioxide. Testing in argon is next, along with high-temperature trials in various media. Evaluation of samples immersed in liquid metal or molten salt is planned for 2020, according to Tucker. Eventually, tests will be conducted in a radiation environment.

The intent, ultimately, is to create a system that can be used to evaluate any material considered for reactor construction. The researchers also expect that there will be a role for the diagnostic tools they develop in non-nuclear applications involving molten salts and liquid metal, such as concentrated solar power plants, as well as in existing test reactors.

“In situ mechanical testing is difficult. I haven’t seen much of it done using these challenging coolants,” Tucker said. “This project is all about collecting data and getting a signal out. With the ability to reliably do that, researchers can dig deeper into how materials behave in various reactor environments, and that will support innovation as reactor designs continue to evolve.”

Sept. 3, 2019