Study mapping U-10Zr porosity informs metallic fuel design

Jossou

According to MIT, the researchers used samples extracted from fuel used in the Fast Flux Testing Facility (FFTF) reactor, which operated from 1982 to 1992 in Washington state. In experiments conducted at Brookhaven National Laboratory, the team used high energy synchrotron X-ray computed tomography to analyze the pore networks and chemical changes that formed under irradiation.

"This study helps us model the pore distribution in the fuel more accurately," said Ericmoore Jossou, a professor at MIT. "It also helps us design for the safe operation of metallic fuels in reactors by giving us a better understanding of the role of pores and their importance."

In an earlier study published in 2020, a Purdue-led research group conducted the first three-dimensional characterization of irradiated U-10Zr. The team studied a tiny sample extracted from a FFTF fuel pin and determined that 7.2 percent of the fuel specimen was porous and found three distinct uranium phase regions: poor, intermediate, and rich.

The new study builds on this work by examining multiple samples from across the full cross section of the fuel, selecting four zones with different quantities of zirconium. By bombarding the samples with high-energy X-rays and observing how interaction with the sample changes them, the researchers were able to reconstruct the fuel's internal pore networks in three dimensions, revealing how porosity, chemistry, and fuel-cladding interactions evolve across the fuel radius.

According to MIT, the researchers found porosity increased modestly from the center of the fuel toward the fuel edge, but pore density jumped by over two orders of magnitude at the fuel's edge by the cladding. The researchers also characterized the size and shape of pores, finding small pores at the center that turn into larger pore networks pointing outward toward the edge.

According to Jossou, fission products, including gases and lanthanides, can migrate to the cladding and react with it, causing embrittlement and damage the fuel system, and pore networks can facilitate this movement. But, he said, this study shows that at high temperatures, porosity can also act as a pathway for the flow of liquid sodium, improving heat transfer, and connected pores can serve as releasing channels for fission gases, reducing the internal fuel matrix stress.

"The pores are currently modeled as spheres; however, in reality they are more complex, especially when many pores merged together," said MIT postdoctoral associate Anthony Harrup. "That's true from the center all the way to the cladding. It explains why the cladding reacts the way it does, and why we see cladding chemicals in the fuel."

Overall, the amount of porosity and how it's distributed is an important factor in understanding a fuel's performance and lifetime. The experimental findings differed from some models of how pores form and how the fuel system swells, MIT said, which could improve simulations to help keep reactors running for longer.

"With this study, we've conducted an in-depth analysis enabled by advanced computational imaging methods that has never been done before, with correlations between local chemical environments and the formation of pores," said Harrup. "It turns out that whether the environment is uranium rich or zirconium rich impacts the morphology and the channels of the pores. That has never been reported before."

Dong Liu, a professor at Oxford University who was not associated with this work, said the study's correlation of three-dimensional porosity to the thermal properties of the fuels is impressive.

"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, a researcher at INL.