A study published Aug. 13, 2026 reported a detailed three-dimensional analysis of irradiated uranium alloyed with 10 percent zirconium by weight, known as U-10Zr, a metallic fuel tested in historic sodium-cooled fast reactors and again drawing interest for advanced reactors. MIT researchers, with Idaho National Laboratory (INL), used high-energy synchrotron X-ray computed tomography at Brookhaven National Laboratory on samples that had been used in the Fast Flux Testing Facility reactor, which operated from 1982 to 1992.

The researchers found porosity increased modestly from the fuel center toward the edge, but pore density jumped by over two orders of magnitude at the edge near the cladding. Small central pores, in their account, give way to larger pore networks pointing outward; those networks permit fission products and lanthanides to move while slowing heat transport. The team also linked pore shapes to local uranium- or zirconium-rich chemistry, and the results differed from some models of pore formation.

Senior author Ericmoore Jossou said the work could improve pore-distribution modeling and safe metallic fuel design, noting that at high temperature connected pores may aid liquid sodium flow and release fission gases. The work was supported by the U.S. Department of Energy Office of Nuclear Energy.