Samantha Gateman and her team at Western University use highly localized electrochemical techniques to see how corrosion starts and evolves at the microscopic level.
Their research focuses on how nuclear fuel and reactor materials change over time, and how those changes could affect the containers and components used to safely store and dispose of them for millennia.
So, when Gateman, Western's NWMO Chair in Radiation Induced Chemistry, and postdoctoral researchers Soroosh Hakimian and Shamim Pourrahimi were given tiny TRISO (tri-structural isotropic) particles - poppy seed-sized nuclear fuel kernel surrounded by four protective layers - to characterize, they approached them the way they approach many unfamiliar materials. First, figure out what is there. Then, determine how it behaves.
To their surprise, they found small regions of silicon inside the TRISO fuel kernel, and that silicon was already there before the particle ever entered a reactor.
TRISO is among the most robust nuclear fuel designs ever developed, with each tiny fuel kernel surrounded by multiple layers designed to contain radioactive fission products.
Researchers had previously found silicon inside irradiated, or used, TRISO fuel that had been exposed to extreme temperatures and radiation. It was generally attributed to silicon migrating inward from the surrounding silicon-carbide protective layer during reactor operation.
Gateman, Hakimian and Pourrahimi have now shown that some silicon is present before the fuel ever enters a reactor, giving researchers a new baseline for understanding what happens to the material during reactor operation and raising new questions about how the material entered the fuel kernel in the first place.
"It was like we were detectives on an episode of CSI," said Gateman, a chemistry professor in Western's Faculty of Science. "At first, we saw the silicon and thought, 'okay, that's interesting. It's a little weird that it's there, but let's keep going.' We had other things we wanted to understand about this material. But then we started asking where that silicon came from, and that's when the investigation really began."
The result, published in the Journal of Nuclear Materials, offers a new clue for understanding one small but important feature of TRISO nuclear fuel.
Each TRISO particle contains a tiny uranium oxycarbide fuel kernel roughly 300 to 400 micrometres across, about the size of a poppy seed, surrounded by layers of carbon and silicon carbide. Hundreds of thousands, potentially millions, of these particles can be packed into a nuclear reactor.
In her lab, Gateman was studying the TRISO fuel particles as part of a larger project to characterize their chemistry for future safe disposal and eventually examine how they may dissolve electrochemically. Before those experiments could begin, her team needed to know exactly what they were working with.
Much of the detective work fell to Hakimian and Pourrahimi, who examined the tiny particles and helped identify the chemical signatures that led to the silicon discovery.
"It was really the carefully detailed research of Soroosh and Shamim that allowed us to follow that trail and figure out the silicon was already there," said Gateman.
Critical to their process was examining fuel that had never been irradiated. By looking at the material before it entered a reactor, the researchers could separate what was present in the fuel kernel from the start, from what might develop during reactor operation.
Samantha Gateman (Christopher Kindratsky/Western Communications)
"We found silicon in the fuel kernel before it had ever been inside a reactor. That means when we see silicon inside irradiated fuel, we can't automatically assume it migrated there from the silicon-carbide coating," said Gateman.
That distinction sent the team back to the fuel-making process.
TRISO fuel kernels are produced through a process called sol-gel fabrication, in which uranium-containing material is formed into tiny spheres in a hot silicone-oil bath before undergoing additional thermal processing and receiving their protective coatings.
"The surprising part was realizing the silicon could be connected to the way the fuel is made," said Gateman. "From a chemistry perspective, the composition of the oil matters because the material is interacting with it as the fuel kernel is being formed."
The researchers had an unusual opportunity to test that idea because they had access to TRISO particles at different stages of fabrication. They could examine the bare kernels, as well as fully coated particles. Silicon was present at both stages.
"We found that the silicon present is not a consequence of the subsequent coating process," said Gateman. "It's already there as you make the kernel."
The findings are particularly interesting because TRISO fuel is not new technology. The concept dates back to the 1950s, and the fuel has been studied for decades. What has changed is the ability to see and chemically characterize materials at increasingly fine scales.
"When we went back through the scientific literature, we found that silicon inside TRISO kernels had been reported before for fuel that had been used during reactor operation. But we couldn't find a detailed measurement of the fuel before it went into the reactor that told us how much silicon was there to begin with," said Gateman. "Now we have the tools to see much more clearly what's actually in these tiny fuel particles. That baseline is important because if we want to understand what happens to these materials in a reactor, we need to know what they looked like chemically before they went in."
Fully ceramic microencapsulated (FCM) fuel pellets contain TRISO fuel particles (USNC)
The finding has already generated interest among researchers working on TRISO fuel.
At an international TRISO fuel workshop this summer, Hakimian and Pourrahimi met authors of earlier work describing silicon-rich phases inside irradiated fuel. Those researchers told Gateman's team they had gone back to their original data and had not found silicon in their samples before irradiation. They continue to believe silicon can migrate into the kernel from the surrounding silicon-carbide layer.
Gateman does not dispute that possibility. Instead, her lab's work adds another piece to the puzzle.
"If we want to know how much is moving during reactor operation, we need to know how much is there in the first place," she said.
Whether the manufacturing process for TRISO fuel should eventually be changed is a different question. Previous research found silicon-rich regions in irradiated fuel could potentially influence gas buildup and pressure within the fuel particle.
"We don't know yet what effect these silicon-rich phases have on fuel performance, but there are some interesting possibilities," said Gateman. "The first step is recognizing they're there and making sure we account for them when we study how the fuel behaves."
