US scientists unlock secrets of nuclear fuel that will power next-generation reactors

Researchers at the Massachusetts Institute of Technology (MIT), alongside those at the Idaho National Laboratory (INL), have conducted the most detailed three-dimensional analysis of uranium fuel and found a hidden pore network inside it. The findings provide new insights into how the fuel works inside the reactor and will help reactors run longer in the future. Nuclear fission is an extensively studied process that we use regularly to extract large amounts of energy from. However, the exact mechanisms of how nuclear fuels work remain unknown. This remains true even for metallic fuels like uranium alloyed with 10 percent zirconium (U-10Zr), the most extensively studied metallic fuel in sodium-cooled fast reactors. As U-10Zr is gaining popularity as a potential fuel for the next generation of advanced nuclear reactors, researchers decided to study it further and understand the exact changes it undergoes inside the fission reactor. They teamed up with INL and Brookhaven National Laboratory (BNL) to arrive at new insights into how it interacts with the fuel cladding. Nuclear fuel and cladding Inside the sodium-cooled fast reactors, nuclear fuel is placed in the form of rods and sealed inside tubes called cladding. Inside each rod, the heat moves outward from the center and reaches the cladding, where it is collected by liquid sodium and carried to generate power. During the reactor operation, the contact between fuel cladding and the fuel results in chemical reactions that generate certain gases or rare earth elements called lanthanides, which cause the cladding to become brittle and damage the fuel system. Using fuel samples from the Fast Flux Testing Facility reactor that were used between 1982 and 1992, the researchers used an approach called high-energy synchrotron X-ray tomography to reconstruct the fuel’s internal pore networks in three dimensions to understand how porosity, chemistry, and fuel-cladding interactions occurred across the fuel radius. In their work, the researchers found that even though the porosity of the fuel increased modestly, the pore density jumped by over two orders of magnitude by the cladding. The pore networks are important since they allow fission products and lanthanides to move but also slow down heat transport. This affects both the performance and the lifetime of the nuclear fuel. Towards future reactors “The ability to directly visualize pore connectivity and fuel cladding interaction in three dimensions gives us important insight for improving fuel performance for advanced metallic fuel for sodium fast reactors,” said Tiankai Yao of INL, who was involved in the work. Since the experimental findings are different from previously proposed models about how pores are formed, the researchers are hopeful that they will aid in simulations to help reactors run longer in the future. “At high temperature, the pores are not all bad, because we found they act as pathways for liquid sodium metal to flow through the fuel and sustain thermal conductivity,” explained Ericmoore Jossou, a professor of Nuclear Science and Engineering at MIT, in a press release. “Connected pores could also serve as releasing channels for fission gases, which reduce the internal fuel matrix stress.” The findings will also help design better fuel systems for next-generation sodium fast reactors. The research findings were published in the journal Materials Today. Get the latest in engineering, tech, space & science - delivered daily to your inbox.Ameya is a science writer based in Hyderabad, India. A Molecular Biologist at heart, he traded the micropipette to write about science during the pandemic and does not want to go back. He likes to write about genetics, microbes, technology, and public policy.