Researchers at the Department of Energy’s Oak Ridge National Laboratory (ORNL) have, for the first time, used neutron scattering to peer inside next-generation TRISO nuclear fuel particles containing high-assay, low-enriched uranium (HALEU). This novel analysis provides critical baseline data on internal fuel chemistry, a key step for developing safer, more efficient high-temperature gas reactors (HTGRs).
Imagine trying to analyze the precise chemical changes inside a reactor core the size of a poppy seed. That’s the scale of the challenge scientists face with tristructural isotropic (TRISO) fuel, a robust particle form crucial for future advanced nuclear reactors. A cross-disciplinary team at ORNL has now cracked this problem, employing the powerful Spallation Neutron Source (SNS) to gather the first-ever neutron scattering measurements on unirradiated HALEU TRISO particles. Reported by the lab, this work illuminates the hidden chemistry that dictates fuel performance and safety.
About the Product, this research tackles a pivotal engineering challenge for next-generation reactors. TRISO fuel particles are tiny, layered spheres designed to be exceptionally durable, containing fission products even under extreme conditions. Their adoption is key for HTGRs, which promise improved safety and efficiency. The Basic Function of the team’s investigation is to establish a precise chemical baseline for these particles before they are irradiated in a reactor. Specifically, they are measuring the initial ratio of uranium carbide to uranium oxide inside the particle’s kernel, as this composition evolves during nuclear fission and critically impacts the fuel’s long-term stability.
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Historically, HTGRs used kernels made from uranium oxide. However, fission liberates oxygen, which can react with the particle’s carbon layers to form carbon dioxide, risking over-pressurization and corrosion. The modern solution, developed under the DOE’s Advanced Gas Reactor (AGR) Fuel Program, uses uranium carbide kernels. Here, the uranium carbide acts as a “getter,” absorbing liberated oxygen to form more uranium oxide, thereby preventing dangerous gas buildup. The Innovator & Engineer behind this analytical breakthrough is the collaborative team led by R&D staff member Will Cureton in ORNL’s Particle Fuel Forms Group. “This collaboration aims to figure out how much of that uranium carbide is consumed after irradiation,” Cureton said. “We know that both uranium carbide and uranium oxide are important to fuel performance, but the exact composition requirements are still unknown.”
To uncover these requirements, the team faced a significant Limitation: the extreme small size and density of the TRISO particles make their internal chemistry nearly impossible to probe with conventional methods. Their solution was to harness ORNL’s world-leading neutron capabilities. At the SNS, they aimed a highly focused, one-millimeter pulsed neutron beam at a batch of particles. As neutrons scatter off the uranium atoms, a detector captures the pattern, revealing the material’s crystalline structure and phase composition without destroying the sample.
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This successful experiment is more than a technical milestone. The Summary of its value lies in paving a new path for nuclear fuel development. By understanding exactly how the kernel composition changes with radiation exposure (“burnup”), scientists can refine fuel fabrication for optimal performance and safety. This foundational knowledge, Cureton notes, can “ultimately enable improvements to the economics of TRISO fabrication methods, leading to safer, more efficient and cost-effective fuel technologies.” It represents a critical step from empirical design to precisely engineered nuclear fuel for a new energy era.
The work, sponsored by the DOE’s Office of Nuclear Energy, involved experts from across ORNL, including Thomas Copinger, Ahmad Mitoubsi, and Katherine Montoya. The next phase will apply this technique to irradiated particles, directly observing the complex phase changes that occur during reactor operation and turning a microscopic poppy seed into a window for macro-scale energy progress.
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