Researchers from Canada found silicon inside TRISO, a robust nuclear fuel material, before the pellets reached a reactor.
The finding came from work by Professor Samantha Gateman and her team at Western University, who were studying TRISO fuel particles as part of a larger project to characterize the material’s chemistry.
The study aimed to examine the chemistry of the nuclear fuel material to help guide safer future disposal and understand how it breaks down over time through chemical reactions.
Silicon found before irradiation
To determine where rust or degradation of the fissile material begins and how it spreads over time, postdoctoral researchers in Gateman’s lab, Soroosh Hakimian and Shamim Pourrahimi, began by examining the material’s composition.
TRISO are tiny poppy seed-sized uranium fuel pellets encased in a protective coating. In their experiments, the team found silicon in the kernels before irradiation.
”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,” Gateman said.
Researchers said this was surprising, as TRISO is considered among the most robust nuclear fuel designs ever developed. Each tiny fuel kernel is surrounded by multiple layers designed to contain radioactive fission products.
Previous research had indicated silicon in TRISO fuel after exposure to extreme temperatures and radiation inside the reactor. In those studies, silicon migration from the surrounding silicon carbide protective layer during reactor operation was suggested as the plausible source of the finding.
However, the latest findings by Gateman and her team showed that silicon is present before the fuel ever enters a reactor.
Fuel fabrication introduces silicon
According to the researchers, this sets a new baseline for understanding what happens to silicon during reactor operation. It also raises new questions about how the material entered the fuel kernel in the first place, Gateman said in a statement.
Gateman said this distinction, finding silicon in TRISO particles before irradiation, sent the team back to the fuel-making process.
The fissile material is generated through a process called sol-gel fabrication. In this procedure, uranium-containing material is formed into tiny spheres by dipping them into a hot silicone-oil bath. These dip cycles take place before the particles are subjected to any thermal processing and receive their protective coatings.
To examine whether silicone particles entered during fabrication, the team studied the fuel particles in both bare and fully coated states. They found silicon at each stage, which astonished the researchers.
”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.”
Researchers inferred that the silicon present in TRISO is not a consequence of the subsequent coating process; rather, it is present during the fabrication of the nuclear kernel.
These results were published in the Journal of Nuclear Materials.