Nuclear fuelUnderstanding next-generation nuclear fuel

The long search for the location of a rare element within nuclear fuel particles has ended. Researchers have finally pinpointed where silver congregates inside irradiated particles of a new type of nuclear fuel.

The finding will help scientists better understand how select fission products such as silver can escape from tristructural isotropic (TRISO) fuel. This specialized fuel could run high-temperature gas-cooled reactors (HTGRs) that have numerous enhanced safety features.

“This is a major achievement for our research in TRISO particle fuel,” says David Petti, Idaho National Laboratory’s director of the Very High Temperature Reactor Technology Development Office.

“Understanding the behavior of fission products in our fuel is critical because of the TRISO coating’s containment function in the overall safety strategy for HTGRs.”

An INL release reports that next-generation HTGR designs incorporate safety systems that rely on the natural laws of physics more than mechanical systems or human intervention. These safety systems extend all the way down to the design of the fuel itself.

For nearly ten years, INL researchers have been studying TRISO fuel, a spherical particle with uranium dioxide or uranium oxycarbide at its core. The core is coated with layers of carbon and silicon carbide — the TRISO coating — which acts as “the primary containment” for fission products. The coated particles are about the size of a poppy seed.

Researchers had known silver fission products were amassing somewhere inside the coated particles.

Silver is one of the few fission products that can migrate outside the particles, and scientists want to better understand such movement. They had not been able, however, adequately to “see” inside the particles until now. The research team reached the new milestone by spotting a tiny sliver of silver using an extremely powerful microscope at the Center for Advanced Energy Studies (CAES). 

Scanning Transmission Electron Microscope (STEM) examination enabled 1-nanometer magnification, where the silver was identified and confirmed in the particle’s silicon carbide layer. Specifically, the silver resided along the “grain boundaries,” the place in a material where one crystal lattice ends and another begins. The STEM microscope helped confirm both the presence and location of the silver.

“The technique and scale of these measurements on TRISO fuel, as well as the identification of silver, are first-of-a-kind and are helping obtain a better understanding of the transport mechanisms associated with these fission products,” says Isabella van Rooyen, INL nuclear materials scientist and principal investigator for the research effort. For the past