As the world gets serious about transitioning to carbon-free energy, one option gaining more interest is nuclear energy such as generation-IV nuclear reactors. Compared to their aging counterparts, these new designs are more efficient and safer. But when it comes to their materials, they do have major drawbacks: They run at higher temperatures, produce higher radiation doses, and create extremely corrosive environments.
Making next-gen nuclear a reality means developing new structural metals that can withstand these more extreme conditions. That is something Hyunseok Oh, an assistant professor of materials science and engineering at the University of Wisconsin-Madison, will research as part of five-year Department of Energy early career research program award.
“At intermediate temperature conditions, around 300 to 500 Celsius, a lot of nuclear materials, like steel for reactor pressure vessels, form small irradiation defects, like dislocation loops,” says Oh. “This leads to the embrittlement of the materials, which can lead to failure. So my proposal is to tackle the embrittlement problem at these temperatures.”
To do so, Oh is looking at complex-concentrated alloys, which are metallic alloys with multiple principal elements in high concentrations—for example, titanium-zirconium-niobium—in which the metals are mixed in equal proportions. These alloys are good at resisting irradiation and embrittlement because their lattice structures are distorted, making it more difficult for irradiation-driven defects to form in the metals.
Oh is focusing on an aspect of these alloys called local chemical ordering, in which 1- to 3-nanometer particles with a special lattice structure form. In most metals, development of these particles often leads to embrittlement. However, in previous research, Oh found that in complex-concentrated alloys, these particles are beneficial and in fact are associated with an increase in plasticity.
So Oh is devoting his attention to complex-concentrated alloys that develop these particles to reverse the mechanical impacts of destructive irradiation loops when exposed to heat and radiation. “The question we’re asking is how can we change the nanostructure or microstructure of these materials to trigger such beneficial mechanisms from nuclear defects rather than embrittlement mechanisms from the nuclear defects,” he says.
To answer that, he is going to design and create many complex-concentrated alloys with local chemical ordering and subject them to radiation, studying them before and after using a battery of advanced characterization tests, including synchrotron-based analysis, high-resolution transmission electron microscopy, atom-probe tomography, and in situ heating and deformation tests inside scanning electron microscopes.
Oh says that to investigate all the materials he wants to, this project will be high-throughput. Working with Adrien Couet, an associate professor of nuclear engineering and engineering physics, he plans to use an ion-beam accelerator to irradiate 25 alloy samples at a time at various temperatures, automatically. He’ll also collaborate with Paul Voyles, the Harvey D. Spangler Professor in materials science and engineering and an expert in atomic-scale transmission electron microscopy. “With that, we can literally see each atom column and the strain there,” says Oh. “So that will be very helpful for understanding this mechanism.”
While understanding how these mechanisms operate is important on a fundamental level, Oh says ultimately he hopes this work can guide the development of new complex-concentrated alloys that will show up in advanced nuclear energy systems in the near future.
Top photo of Hyunseok Oh by Joel Hallberg