It’s a new type of nuclear reactor core.
And the materials that will make it up are novel — products of Oak Ridge National Laboratory’s advanced materials and manufacturing technologies.
With a tight timeline, researchers are tasked with ensuring the novel materials can stand up to the intense heat, pressure and neutron bombardment inside a reactor core. How will they make sure these materials can do everything they need to do as crucial parts of the Transformational Challenge Reactor?
By testing them in an experimental reactor built in the 1960s.
The research team will irradiate three components in ORNL’s High Flux Isotope Reactor: 3D-printed silicon carbide, 3D-printed 316L stainless steel, and yttrium hydride, the building blocks of the gas-cooled reactor core. Together, these materials will drive what ORNL envisions as the reactor of the future.
“This project aims to demonstrate how advanced manufacturing can be applied to nuclear power by combining 3D-printed materials and an innovative reactor design and then making it go critical in less than five years,” said Kory Linton of ORNL’s Nuclear Fuels and Materials Group.
HFIR is distinctly suited to aiding with this task, he said.
“A lot of the test reactors that are available would not lend themselves to an agile project like this with a tight timeline,” Linton said. “ORNL is uniquely positioned for irradiation testing in support of the Transformational Challenge Reactor because HFIR is such a powerful reactor capable of fast-turnaround, small standardized experiments.
“Within a year, we can have materials in HFIR. Within another year, we can have data. For the irradiated materials community, that’s a very quick turnaround time.”
The fuel matrix for the Transformational Challenge Reactor will be made of silicon carbide, a chemical compound of carbon and silicon that produces a high-quality, durable ceramic that’s also an excellent conductor of heat.
Silicon carbide’s durability and versatility have already ensured its usefulness in numerous technical applications, including nuclear projects, and a lot of data exist on its performance. But ORNL’s silicon carbide, or SiC, is different because it’s 3D printed.
Using a type of additive manufacturing, in which powder is layered with a binding agent to form a solid matrix, SiC can be printed in unusual shapes and sizes.
“You can put it into geometries that historically you would not even consider,” Linton said.
Linton and his team already have completed irradiating thin discs of 3D-printed SiC at different temperatures. A holder containing these discs is placed in a sealed capsule with very tight design tolerances that provide small gas gaps to manage the temperature during irradiation. The capsule is then inserted into the center of HFIR for irradiation.
After irradiation at target temperatures ranging from 400 to 900 degrees Celsius, the discs are being checked for strength and swelling and analyzed to determine how quickly the 3D-printed SiC transfers heat. Those parameters will be used to inform TCR’s core design, ensuring the most relevant materials property data are captured.
The core structure
3D-printed 316L-grade austenitic stainless steel, an alloy shown to withstand high temperatures for long periods without corrosion, will be used for the lattice structure inside TCR’s core. This structure will hold the fuel rods in place. The stainless steel will also be used for some bristles and springs.
ORNL’s 316L stainless steel is made by printing layers of powder that are fused together as they are created. It has undergone neutron irradiation along with samples of traditionally wrought 316L to compare how they perform in temperatures ranging from 300 to 600 degrees Celsius and at varying doses of radiation.
But printing it layer by layer could also produce variations in density, chemistry, porosity and other characteristics, Linton said. That’s why, when he cuts a sample for testing, he’s very careful to note its exact location within the original printed geometry.
Learning which printing patterns produce the most desirable material will, in theory, allow researchers to “teach” a computer the ideal traits. Then, through artificial intelligence, it will compute the best methods for printing them.
“Each print provides a rich amount of data that can be used to continually improve the printing process,” he said.
If additively manufactured 316L performs as well as or better than wrought steel, Linton said, it will save time and money in fabrication of parts, and it will also allow for additional design flexibility.
“The promise of advanced manufacturing is that you’re going from a slower and less-flexible environment to where you can come up with a design, prototype it, see how it performs, make adjustments and try again,” Linton said. “You’re moving through and testing ideas very quickly.”
Yttrium hydride, a compound of a transition metal and hydrogen, will be used in TCR’s nuclear core as a moderator, which is the substance that slows down neutrons during fission.
The idea that YHx could be an effective core moderator dates back to early gas-cooled reactor designs, but data on the compound are scarce, as it was not widely commercially available in the past.
Now that yttrium is more readily available, YHx has been selected for TCR because of its high hydrogen atom density and exceptional ability to withstand high temperatures without changing its chemical composition.
“TCR is a ripe opportunity to go back to a material that the market has changed on,” Linton said.
Capsules of YHx will be irradiated in HFIR at 600 and 900 degrees Celsius at three different radiation doses. In addition, specimens will contain varying amounts of hydrogen so that researchers can see if that makes a difference in heat capacity, durability, expansion and how quickly it transfers heat.
“Considering the potential for broad application in advanced reactors, this is a cutting-edge experiment,” Linton said.
HFIR, history help
Linton said ORNL is uniquely suited for these experiments not only because of the availability of HFIR — a DOE Office of Science user facility — but also because so much work has already been done here to design irradiation vehicles and quickly move these materials through post-irradiation examinations. Years of research and innovation come into play when deciding the most effective ways to measure these new materials, he said.
“This is the ultimate challenge of a project: In the end, you’re going to build a reactor and control a chain reaction,” Linton said. “You get to take energy and experience from across the lab, across many different disciplines and put them together.
“This is what you picture a national lab doing – and it’s exciting.”
The Transformational Challenge Reactor is supported by DOE’s Office of Nuclear Energy.
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.