Oak Ridge National Laboratory is reporting a development in TRISO fuel research that could help evaluate high-temperature gas reactor fuels. ORNL researchers used the Spallation Neutrons and Pressure Diffractometer at the lab's Spallation Neutron Source to make neutron scattering measurements on TRISO fuel particles containing high-assay low-enriched uranium (HALEU).

Special kernels: The research reported by ORNL is part of a collaboration among different scientific teams at the lab that is investigating TRISO kernels developed specifically for the Department of Energy's Office of Nuclear Energy Advanced Gas Reactor (AGR) Fuel Qualification and Development Program. These TRISO kernels have a multiphase mixture of uranium oxide and uranium carbide contained by several layers designed to maintain their structural integrity and "to address deleterious behavior from oxygen liberation," according to ORNL.

The oxygen generated by the fission of uranium oxide in typical 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," ORNL explained. However, in the uranium carbide-containing kernels, "the uranium carbide component is consumed by absorbing the liberated oxygen, producing uranium oxide, instead of carbon dioxide."

ORNL researchers want 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 member in the lab's Particle Fuel Forms Group.

Pulsed neutron beam: To gain insights into this fuel phenomenon, the researchers aimed a tightly focused, 1-millimeter pulsed neutron beam from the diffractometer at unirradiated HALEU-containing TRISO particles from the AGR Fuel Qualification and Development Program. As the beam passed through the TRISO particles, some neutrons were absorbed by the uranium and others were scattered. A neutron area detector collected the scattered neutrons, yielding data about the TRISO properties and composition.

ORNL considers these data to be "early measurements [that] offer a path for using neutron scattering with reactor-irradiated TRISO particles." Further research with this neutron scattering technique will allow investigators to "characterize the complex phase changes that occur within TRISO particles, opening the door to new scientific insights and research directions."

Improvements to economics: Cureton said that the "experiments offer valuable insight and lay the groundwork for more detailed modeling of TRISO fuels. 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."