Study Reveals Three-Dimensional Pore Networks Inside Uranium-Zirconium Metallic Nuclear Fuel

Researchers at the Massachusetts Institute of Technology (MIT), in collaboration with Idaho National Laboratory (INL), conducted three-dimensional imaging studies on irradiated uranium-10 zirconium metallic fuel, revealing the internal pore networks within the fuel and their relationship with chemical changes and fuel-cladding interactions. The researchers believe these results will help improve metallic fuel performance models, providing references for extending the operating cycles of certain reactors and for the design of next-generation sodium-cooled fast reactor fuel systems.

Uranium-10 zirconium (U-10Zr) is a uranium-zirconium alloy metallic fuel containing 10% zirconium, which has been extensively tested in sodium-cooled fast reactors such as Experimental Breeder Reactor II (EBR-II) and the Fast Flux Test Facility (FFTF). With the advancement of advanced reactor research and development, this type of metallic fuel has regained attention, but the details of pore formation, swelling, heat transfer, and interactions with cladding materials under reactor irradiation conditions still require further clarification.

During reactor operation, heavy atoms within the nuclear fuel undergo fission, producing fission products and inducing material defects. These defects affect fuel swelling, heat transfer, and chemical migration. For sodium-cooled fast reactors, fuel is typically encapsulated in metallic cladding, and chemical interactions between the fuel and cladding may cause fission gases, rare earth elements, and other species to migrate toward the cladding, thereby affecting cladding integrity and fuel system lifetime.

The fuel samples used in this study were obtained from the Fast Flux Test Facility. Idaho National Laboratory was responsible for sample management and preparation, and the research team subsequently used high-energy synchrotron X-ray computed tomography at Brookhaven National Laboratory (BNL) to perform three-dimensional reconstruction of the irradiated U-10Zr fuel, observing changes in porosity, pore morphology, chemical composition, and fuel-cladding interaction along the fuel radius.

The results show that fuel porosity increases slightly from the center to the edge, but near the cladding region, pore density jumps by more than two orders of magnitude. The researchers also found that the fuel center region is dominated by smaller pores, and as the edge is approached, pores gradually evolve into larger, more complex, and interconnected network structures. This differs from previous models that simplified pores as spheres.

Anthony Harrup (安东尼·哈鲁普), the paper's first author and a postdoctoral researcher at MIT, stated that the actual pore structure is far more complex than spherical models suggest, especially after multiple pores merge. This structure helps explain how the cladding reacts and why cladding-related chemical components can be observed within the fuel interior.

The study also linked pore morphology to local chemical environments. The team found that uranium-rich and zirconium-rich regions influence pore morphology and connectivity. The pore networks near the fuel edge may, on one hand, provide pathways for the migration of fission products and lanthanides, and on the other hand, alter heat transfer processes, thereby affecting fuel performance and service life.

Eremore Jossou (埃里克摩尔·乔苏), Professor of Nuclear Science and Engineering at MIT, stated that this research helps more accurately simulate pore distribution in fuel and further understand the role of pores in the safe operation of metallic fuel. The research team believes that under high-temperature conditions, interconnected pores do not necessarily have only negative effects; they may provide channels for liquid sodium to flow within the fuel, helping maintain thermal conductivity, while also serving as pathways for fission gas release, reducing internal stress within the fuel matrix.

This research was funded by the DOE Office of Nuclear Energy and utilized relevant resources at Brookhaven National Laboratory and Idaho National Laboratory. The researchers stated that if future models can incorporate three-dimensional pore topology, it will help more accurately assess the thermal performance, cladding compatibility, and long-term operational behavior of sodium-cooled fast reactor metallic fuel.