Tantalum-based coating makes fusion reactors safer

“The fusion community is urgently looking for new manufacturing approaches to economically produce large plasma-facing components in fusion reactors,” Mykola Ialovega, a postdoctoral researcher in nuclear engineering and lead author of the paper, said in a media statement. “Our technology shows considerable improvements over current approaches. With this research, we are the first to demonstrate the benefits of using cold spray coating technology for fusion applications.”

In detail, Ialovega and his team used a cold spray process to deposit a coating of tantalum, a metal that can withstand high temperatures, on stainless steel. They tested their coating in the extreme conditions relevant to a fusion reactor and found that it performed very well. Importantly, they discovered the material is exceptionally good at trapping hydrogen particles, which is beneficial for compact fusion devices.

“We discovered that the cold spray tantalum coating absorbs much more hydrogen than bulk tantalum because of the unique microstructure of the coating,” Kumar Sridharan, senior author of the study, said. 

Over the last decade, Sridharan’s research group has introduced cold spray technology to the nuclear energy community by implementing it for multiple applications related to fission reactors.

“The simplicity of the cold spray process makes it very practical for applications,” Sridharan said.

In fusion devices, plasma—an ionized hydrogen gas—is heated to extremely high temperatures, and atomic nuclei in the plasma collide and fuse. That fusion process produces energy. However, some hydrogen ions may get neutralized and escape from the plasma.

“These hydrogen neutral particles cause power losses in the plasma, which makes it very challenging to sustain a hot plasma and have an effective small fusion reactor,” Ialovega said.

This is why the researchers set out to create a new surface for plasma-facing reactor walls that could trap hydrogen particles as they collide with the walls.

Tantalum to the rescue

Tantalum is inherently good at absorbing hydrogen—and the researchers suspected that creating a tantalum coating using a cold spray process would boost its hydrogen-trapping abilities even more.

Creating a cold sprayed coating is somewhat like using a can of spray paint. It consists of propelling particles of the coating material at supersonic velocities onto a surface. Upon impact, the particles flatten like pancakes and coat the entire surface, while preserving nanoscale boundaries between the coating particles. The researchers discovered that those tiny boundaries facilitate the trapping of hydrogen particles.

Ialovega conducted experiments on the coated material at facilities at Aix Marseille University in France and Forschungszentrum Jülich GmbH in Germany. During these experiments, he found that when he heated the material to a higher temperature, it expelled the trapped hydrogen particles without modifying the coatings—a process that essentially regenerates the material so it can be used again.

“Another big benefit of the cold spray method is that it allows us to repair reactor components on-site by applying a new coating,” Ialovega pointed out. “Currently, damaged reactor components often need to be removed and replaced with a completely new part, which is costly and time-consuming.”

The researchers plan to use their new material in the Wisconsin HTS Axisymmetric Mirror (WHAM). The experimental device is under construction near the city of Madison and will serve as a prototype for a future next-generation fusion power plant that UW–Madison spinoff Realta Fusion aims to develop.