Adding carbon to tri-structural isotropic uranium fuels

When designing the next generation of nuclear reactors, many researchers are turning to tri-structural isotropic (TRISO) fuels. These are uranium-bearing fuel kernels, typically uranium dioxide (UO₂), surrounded by protective layers including pyrolytic carbon and silicon carbide to contain the reaction products. These fuels are cleaner and safer than traditional uranium fuels.

However, under high temperatures, oxygen from the UO₂ reacts with carbon in the surrounding buffer layer to form carbon monoxide gas, which can contribute to internal pressure and coating stresses. Adding carbon to the fuels to form uranium oxycarbide (UCO) can reduce the amount of carbon monoxide released, but may also impact the pellets’ structural properties.

Dhanish et al. used density functional theory to investigate the carbon-containing oxide-rich phase of UCO fuel kernels, examining their crystal and electronic structures, magnetism, chemical bonding, and elastic properties.

“Our calculations show that carbon does not simply make the material uniformly softer or harder,” said author Sidhik Dhanish. “Instead, it changes how the material responds to deformation in different crystallographic directions.”

The authors found that the introduction of carbon results in local variations in chemical bonding and modifies the electronic states of the uranium. It also modifies the directional stiffness of the crystal, making the material more elastically isotropic.

The authors plan to continue their investigations by incorporating irradiation effects into their models, and by integrating their results into more comprehensive models.

“These data can be incorporated into multiscale fuel-performance models that predict stress development, swelling, and structural evolution during reactor operation,” said Dhanish. “Better predictive models ultimately contribute to the design of safer and more reliable advanced nuclear fuels.”

Source: “Carbon-induced magneto-elastic coupling and elastic anisotropy evolution in dilute oxide-rich fluorite UO_2-xC_x for TRISO fuels,” by S. Dhanish, Asmabi Thottathil, Barbara Szpunar, and Jerzy Szpunar, Journal of Applied Physics (2026). The article can be accessed at https://doi.org/10.1063/5.0343756 .