TPMS fuel: INL investigates 3D-printed minimal surfaces for reactor cores

At Idaho National Laboratory (INL), researchers are working on a nuclear fuel concept that combines mathematical minimal surfaces with additive manufacturing. The approach, called INFLUX (Intertwined Nuclear Fuel Lattice for Uprated heat eXchange), replaces the traditional cylindrical fuel rod with a three-dimensional lattice of triply periodic minimal surfaces (TPMS), as known from natural structures or modern heat exchangers. It is only 3D printing that makes this complex geometry technically accessible.

TPMS can roughly be described as sine waves in space whose periodic equations define a continuously curved, repeating surface.

“TPMS is like a sine wave in three dimensions,” said INL researcher Nicolas Woolstenhulme. “The equations that define these things look like terrible trigonometry equations — they define a continuously curved surface that repeats, and you can create a lattice. This lattice will actually create these different volume domains that are intertwined with each other but don’t mix. We said, ‘Hey let’s put nuclear fuel in that.’”

“Cylinders are actually a terrible shape for heat transfer,” Woolstenhulme said. “One of the things that inspired us was seeing what other industries were doing with additively manufactured heat exchangers that mimic complex geometries.”

It is precisely these volumes that the researchers use to accommodate fuel and coolant in a finely structured, tightly coupled arrangement. The goal is significantly improved heat transfer compared with tube bundles from the 1950s.

“We saw heat exchangers that use triply periodic minimal surfaces and said, ‘It’s just perfect,’” Woolstenhulme continued. “It’s nature’s answer to the optimal geometry for nuclear fuel.”

“The bottom line is that this geometry does indeed triple the heat transfer coefficient compared to standard rod-type fuel,” Woolstenhulme said. “That’s a big deal. It has a direct impact on the power density of the fuel rod and thus the economics of a nuclear reactor.”

For initial experiments, the team used 3D printing to produce an electrically conductive polymer composite model of the INFLUX lattice and integrated temperature sensors. The structure was heated by a current pulse while gas and liquid coolants removed the heat. According to INL, the TPMS geometry tripled the heat transfer coefficient compared with rod fuel. This lowers the fuel temperature, enables higher power densities and improves behavior in accident scenarios because the core cools down more quickly after shutdown.

“We need to figure out how to optimize the hydraulic resistance for a given plant design,” Woolstenhulme said. “We need to decide which plant type would benefit from this. We know that any nuclear reactor would benefit from this geometry. The question is, which one benefits most?”

“There are heat exchange geometries in a nuclear reactor other than the core,” Woolstenhulme said. “There are heat exchangers that make steam. There are heat exchangers that cool the water or other coolant. Nuclear deployments of normal fluid-fluid heat exchangers in these geometries would be a really good stepping stone toward nuclear fuel applications.”

“The research we’ve done today proves that the hypothesis is true,” Woolstenhulme said.

Transferring the concept to nuclear-grade materials is, however, laborious. The intricate geometry cannot be produced with conventional processes, which is why INL has combined a process chain of commercial 3D printing and hot isostatic pressing to create ceramic–metal and metallic composite structures. In the long term, the researchers see potential applications in microreactors or gas-cooled concepts, but also in conventional heat exchangers of a power plant.