In nuclear fusion reactors, the nuclei of atoms fuse to generate more energy than is consumed. This energy-producing plasma is roughly the temperature of the sun, which is powered by the same fusion energy principles. The sun, however, does not need to be confined in the way fusion plasma requires on Earth.
Jean Paul Allain, head of the Ken and Mary Alice Lindquist Department of Nuclear Engineering, has received two grants from the U.S. Department of Energy (DOE) totaling $3.6 million to study how to improve the effectiveness and survivability of reactor materials responsible for containing the plasma.
“Finding solutions to the challenges of how to deal with high thermal and particle wall loads is a critical area of fusion research,” Allain said. “The plasma temperature is so high at the boundary of the wall, it will melt any solid. You either need to develop a liquid metal that can’t be damaged or a solid that self-heals.”
To explore the first option, the DOE awarded Allain, in partnership with Princeton University and the Massachusetts Institute of Technology (MIT), $2.5 million over five years. Allain will work with Penn State colleagues in the College of Engineering, the Materials Research Institute and the Applied Research Laboratory to build a porous scaffolding, over which lithium-based liquid metal could move.
“The biggest challenge is how do we deliver the liquid metal into the reactor,” Allain said. “We’re going to mimic how trees get water from their roots to their leaves: capillary force.”
A series of tiny tube-like structures run through the tree. Tension between the water droplets and the surface of the structures propels the water up, against gravity. Allain and the research team will mimic those same principles by using capillary force to drive the liquid metal up a porous but solid scaffold and deliver it to the surface wall of the reactor. As plasma in the reactor heats and touches the wall, ideally the liquid metal will flow over the affected area, immediately healing any damage.
“Liquid metal has issues: how does it hold together, for example?” Allain said. “A huge plasma force could splash the liquid and force the reactor off. Luckily, in this type of reactor, there is not a risk of melt down – the plasma just turns off. Our design of a pore structure should help address this concern.”
Allain’s research team will test different materials and develop small 3D prototypes. Bruce Koel, professor at Princeton University, will lead a team that plans to examine surface interactions on the prototypes, while MIT scientist Kevin Woller and his group will test the prototypes in the conditions found in a plasma reactor. The goal, according to Allain, is to determine the best combination of scaffolding material and lithium liquid metal to scale up for use in reactors.
At the same time, Allain will also explore another option – a solid that self-heals. The DOE awarded Allain $1.1 million over three years to develop a self-healing plasma reactor wall.
“A solid wall is the more obvious plan, as it can hold itself together,” Allain said. “But it could be so greatly damaged by extreme heat fluctuations and neutrons that the structure becomes so weak it fails.”
In collaboration with Xing Wang, assistant professor of nuclear engineering at Penn State, who will also contribute to the first grant project, Allain will develop a tungsten alloy that contains ultrafine particles that identify and restructure to repair damages.
Most damage from the fusion reaction to solid walls involves both surface and deep penetration of subatomic particles that displace the atoms in the wall. At that quantum level, the atoms of the wall – namely, the ultrafine particles dispersed throughout – rearrange to alleviate the weak points in the material. The wall heals itself.
“We have our hypotheses, but we just don’t know how a self-healing solid or a liquid metal structure will actually perform under extreme conditions,” Allain said. “We’ve assembled two great teams, and we’re going to find the answers.”