Building the materials for the next generation of nuclear reactor

Building the materials for the next generation of nuclear reactor
by Julianne Hodges, Texas A and M Engineering
College Station TX (SPX) Nov 05, 2024

Before they can ever safely power up, future nuclear reactors will need to be designed and built from innovative, well-tested materials. Three Texas A and M Ph.D. students and one postdoctoral researcher will take part in supporting this effort with funding through Rapid Turnaround Experiment (RTE) projects.

The RTE program provides researchers with resources at universities and national laboratories through the Nuclear Science User Facilities (NSUF) program. Projects are selected based on scientific merit and the potential to impact nuclear materials research. Each recipient receives between $50,000 and $70,000 from the U.S. Department of Energy Office of Nuclear Energy to cover equipment costs.

Among the 19 recipients who won the RTE program's third call in 2024, three are from Texas A and M: Ph.D. students Rijul Chauhan and Kenneth Cooper and Zhihan Hu, a recent Ph.D. graduate working at Texas A and M as a postdoctoral researcher. Earlier this year, graduate student Benjamin Mejia Diaz also received an RTE project.

"All these projects are highly innovative and expected to create new knowledge or lead to methodology developments," said Texas A and M nuclear engineering professor Lin Shao, who is involved with each of the students' projects. "I am proud of the innovative thinking from the students. I also applaud the NSUF program for supporting students as principal investigators. This brings additional benefits in training and education."

Chauhan, Cooper, Hu and Mejia Diaz will all use the Texas A and M Accelerator Laboratory to irradiate material. They will send their samples to the Center for Advanced Energy Studies at Idaho National Laboratory, where the lab's Focused Ion Beam and Transmission Electron Microscopy facilities will characterize the material and gather data.

Mejia Diaz is developing a new two-step method of irradiating material to more accurately test how it can form small voids and swell.

Under radiation, the particles crashing into the material knock atoms around, which then displaces more atoms, creating vacancies. These vacancies eventually form together, or nucleate, into larger voids. The point of this irradiation process is to replicate the effect of neutrons bombarding the material, as would happen in a nuclear reactor. Typically, this testing uses a beam of heavy ions, such as electron-less iron atoms, which displaces a lot of atoms in the material similar to neutron irradiation. However, heavy ions aren't a perfect surrogate.

Instead, Mejia Diaz proposes first irradiating the material with a proton beam to kickstart the void nucleation process in a more stable way. Then, irradiating the material with iron ions would help those voids grow.

"Using just heavy ions might not be as close as we can get to simulating neutron damage," Mejia Diaz said, "but if we do proton damage before we do heavy ion damage, we can get even closer to simulating what we're interested in by using an accelerator."

Understanding the voids created by the radiation in a nuclear reactor is important because these voids cause swelling. This can be an important factor to consider for reactors that experience more radiation damage or operate for a long time.

"Understanding the voids leads us to understand how the materials properties change: the mechanical properties, the radiation properties, the thermal properties," Mejia Diaz said.

Chauhan proposes a more efficient way to measure how materials swell at different temperatures due to irradiation.

The typical way to test this phenomenon is by using radiation to heat very thin samples to particular temperatures one at a time. This would then be replicated several times to gather data on the conditions being studied.

Chauhan's method would simultaneously test a collection of various samples of different heights. When heated, the bottom of these samples would reach the same temperature, but the tops would heat up differently.

"With this technique, you can combine multiple conditions into one experiment, thus saving a lot of time and making it more efficient," Chauhan said.

New types of reactors will require new materials, Chauhan said, making it crucial to understand the behavior of these materials.

"It's very important that we test these materials thoroughly to make sure that our devices are as safe as possible," he said. "Swelling is one of the major ways that materials degrade inside a nuclear reactor, and it's extremely important that we study all these materials before we put them into a reactor."

Cooper is also investigating how irradiation and corrosion work together to alter the metal alloys that could be used to build molten salt reactors. In this project, he will look at how radiation can speed up or slow down the corrosion process.

Molten salt reactors, a decades-old experimental design that is now popular in research, use certain types of salts as a coolant, such as FLiNaK, a fluoride-based salt. Cooper's project will investigate the corrosive effects of FLiNaK, but his experiment could be adjusted for other salt compounds.

For the material being degraded, his experiment will focus on two metal alloys: 316L stainless steel and Hastelloy X, a mixture of nickel, chromium, iron and molybdenum.

"Seeing how metals perform in corrosion-plus-irradiation environments is imperative to the development of future nuclear reactor materials," Cooper said.

This project will rely on a custom-built target holder that Cooper designed. This apparatus can let a proton beam pass through a thin foil of metal, causing irradiation between the foil and the molten salt sample. This task can be challenging, Cooper said, because the experiment must balance maintaining a vacuum and melting the salt compound with irradiating the metal.

"This research is a void in the field because it's somewhat difficult to do," he said.

Hu will continue his research on HT9, a strong alloy that works well at high temperatures and could be used for advanced reactors.

"My project this time will be to get a more detailed evaluation of how good this HT9 alloy is when used in a reactor environment," Hu said.

HT9's resistance to swelling at high temperatures makes it appealing for the next generation of reactors. During a past experiment, however, Hu and colleagues found that irradiation can cause solid pieces, or precipitates, of carbide to form within HT9 at lower temperatures. In this project, Hu will explore the properties of this precipitate, such as confirming the temperatures at which they appear and disappear. His research will also investigate if the precipitate changes the material's strength.

"The conditions when these carbides form really depends on the type of reactor, and the location of where the material is applied in the reactor," Hu said.

Just like in Hu's previous experiment, the HT9 will be tested between 450 C and 550 C, the range at which the carbide precipitates formed.

"This material is believed to be a good component to be used in the reactor," Hu said. "But if after radiation, the hardness or the tensile property changes, it might be a concern in a reactor after some years of usage."