Neutron scattering reveals hard-to-detect chemistry inside novel reactor fuels

Probing TRISO chemistry for next generation nuclear

Compared with traditional light-water reactor fuel, HALEU enables significantly higher energy production, known as burnup, leading to improved reactor performance - including better fuel utilization, longer intervals between refueling and reduced overall waste generation.

While TRISO particles are designed for robust performance, incorporating HALEU in their fuel design is key to further maximizing the efficiency of high-temperature gas reactors (HTGR) currently under development.

TRISO kernels for the DOE's Office of Nuclear Energy Advanced Gas Reactor (AGR) Fuel Qualification and Development Program are approximately the size of a poppyseed and include a multiphase mixture of uranium oxide and uranium carbide contained by several layers designed to maintain their structural integrity and effectively contain fission products.

Rather than using uranium oxide kernels which were historically used for HTGRs, the AGR program used uranium carbide TRISO kernels to address deleterious behavior from oxygen liberation. Oxygen generated by nuclear fission of uranium oxide in TRISO interacts with the carbon layers and forms carbon dioxide, which can over-pressurize the TRISO particle's layers and corrode the silicon carbide layers. In uranium carbide kernels, the uranium carbide component is consumed by absorbing the liberated oxygen, producing uranium oxide, instead of carbon dioxide.

"This collaboration aims to figure out how much of that uranium carbide is consumed after irradiation, evaluated as a function of the fuel's burnup," said Will Cureton, a R&D staff member in the Particle Fuel Forms Group at ORNL. "We know that both uranium carbide and uranium oxide are important to fuel performance, but the exact composition requirements are still unknown."

Neutron scattering illuminates fuel insights

To gain a peek into the microscopic TRISO particles, researchers spanning ORNL's world-class fission and neutron scattering expertise partnered to establish a pre-irradiation baseline for uranium carbide content. Using the Spallation Neutron and Pressure Diffractometer at ORNL's Spallation Neutron Source, a DOE Office of Science user facility, researchers aimed a highly focused, one-millimeter pulsed neutron beam at an unirradiated HALEU-containing TRISO particles from the same batch of particles irradiated during DOE's Office of Nuclear Energy AGR Program.

As the spallation neutron beam passes through the TRISO particle, some neutrons are absorbed by the uranium while most are scattered away. A neutron area detector collects the scattered neutrons, providing information about the particle's properties and composition.

A boon for future TRISO HALEU nuclear fuel development

These early measurements offer a path for using neutron scattering with reactor-irradiated TRISO particles. Looking ahead, neutron scattering could help researchers characterize the complex phase changes that occur within TRISO particles, opening the door to new scientific insights and research directions.

"These experiments offer valuable insight and lay the groundwork for more detailed modeling of TRISO fuels," said Cureton. "Improved understanding of how kernel composition impacts TRISO behavior can ultimately enable improvements to the economics of TRISO fabrication methods, leading to safer, more efficient and cost-effective fuel technologies."

The experiment's initial success reflects the collaboration and hands-on problem-solving expertise at ORNL. Team members included staff from the Neutron Sciences and Fusion and Fission Energy and Sciences Directorates, including Thomas Copinger, Ahmad Mitoubsi, Antonio Moreira dos Santos, John Hirtz, Jasmine Hinton, Michelle Everett, Stan Cooper, Matthew Tucker, Mason King, Tyler Gerczak, Andy Kercher and Katherine Montoya.

This research was sponsored by DOE's Office of Nuclear Energy through the AGR Program's TRISO research and development efforts as part of the Advanced Fuels Campaign.

ORNL is committed to supporting U.S. energy needs by pursuing strategic research that advances a wide variety of affordable, abundant and competitive nuclear technologies, and strengthens national security. The lab's scientific expertise and world-class facilities are often the first step in advancing nuclear energy innovations.

ORNL is committed to supporting U.S. energy needs by pursuing strategic research that advances a wide variety of affordable, abundant and competitive nuclear technologies, and strengthens national security. The lab's scientific expertise and world-class facilities are often the first step in advancing nuclear energy innovations.

UT-Battelle manages ORNL for the DOE's Office of Science, the single largest supporter of basic research in the physical sciences in the United States. The Office of Science is working to address some of the most pressing challenges of our time. For more information, please visit energy.gov/science. - Liz McCrory