Advanced nuclear fuel layers lose hardness after irradiation, US lab tests show

Researchers at the US Department of Energy’s Oak Ridge National Laboratory have measured the mechanical properties of irradiated HALEU-TRISO fuel particles, giving engineers new data on how the tiny protective layers of advanced nuclear fuel change after irradiation. The measurements focused on individual layers inside TRISO particles, including silicon carbide and inner and outer pyrolytic carbon. Researchers used ORNL’s instrumented indentation system for irradiated materials and nuclear fuels to test particles from the Advanced Gas Reactor Fuel Development and Qualification Program. The tests found changes in hardness and stiffness after irradiation. The silicon carbide layer showed reduced modulus and hardness, while the pyrolytic carbon layers also displayed changes in their mechanical properties. The results matter because the coatings are central to TRISO’s ability to contain radioactive material during reactor operation. Understanding how their mechanical behavior changes with irradiation can help engineers improve models used to predict fuel performance. Testing fuel layer by layer TRISO, or tristructural isotropic fuel, uses a fuel kernel surrounded by multiple coating layers. The particle contains a low-density carbon buffer, inner pyrolytic carbon, silicon carbide and outer pyrolytic carbon. The layered structure is designed to retain fission products and withstand the high temperatures associated with advanced gas reactors. The silicon carbide layer is particularly important because it provides a major barrier against the release of metallic and gaseous fission products. The new work adds mechanical measurements to that picture. Instead of assessing the particle only as a whole, researchers used instrumented indentation to examine the response of its individual coating layers. “These measurements can help compare a particle’s original manufactured condition with how it behaves after irradiation at different temperatures and burnups,” said ORNL’s Katherine Montoya, a R&D associate staff member in the Particle Fuel Forms group. “These data improve nuclear fuel performance models and support the broader adoption of gas reactors.” Irradiation changes fuel behavior The measurements follow neutron scattering work at ORNL earlier this year on HALEU-containing TRISO particles. That effort examined chemistry inside the fuel, while the latest tests focused on mechanical properties. Mechanical properties such as hardness and stiffness can help researchers understand how the protective layers respond to stresses and whether changes could affect fuel performance. The data are particularly relevant as advanced reactor developers work with HALEU, or high-assay low-enriched uranium. HALEU contains more uranium-235 than the fuel used in most existing commercial reactors and is being developed for several advanced reactor designs. TRISO fuel is being considered for high-temperature gas-cooled reactors and other advanced concepts because its coated-particle architecture provides multiple barriers around the fuel. ORNL says understanding how those barriers evolve under irradiation is important for predicting performance. By measuring the irradiated coatings directly, researchers can compare their properties with those of as-manufactured particles and feed the results into fuel-performance models. The measurements could therefore help close a key data gap between how TRISO fuel is made and how it behaves after operating inside a reactor.Get the latest in engineering, tech, space & science - delivered daily to your inbox.With over a decade-long career in journalism, Neetika Walter has worked with The Economic Times, ANI, and Hindustan Times, covering politics, business, technology, and the clean energy sector. Passionate about contemporary culture, books, poetry, and storytelling, she brings depth and insight to her writing. When she isn’t chasing stories, she’s likely lost in a book or enjoying the company of her dogs.