Next-Generation Nuclear Energy: Crafting the Future with Innovative Fuel Designs 

Michael Tonks, Ph.D., and Assel Aitkaliyeva, Ph.D.

As the world grapples with the need for carbon-free power generation, advanced reactor designs are crucial to the transition. A Department of Energy (DOE) Nuclear Energy University Program (NEUP)-funded investigation into the behavior of U-Zr annular fuel will not only address uncertainties in advanced reactor designs but also contribute to the broader goal of establishing nuclear energy as a sustainable and viable solution for the future. 

The $1 million NEUP award to researchers at the University of Florida’s Nuclear Engineering Program within the Department of Materials Science & Engineering supports cutting-edge research and underscores the DOE’s commitment to fostering collaborative efforts between academia and industry. In partnership with TerraPower and Idaho National Laboratory (INL), faculty members Mike Tonks, Ph.D., professor and associate chair in the department, and Assel Aitkaliyeva, Ph.D., associate professor, aim to expedite the development of innovative solutions that could shape the landscape of next-generation nuclear energy. 

The potential implications of this research are profound. Understanding the intricacies of U-Zr annular fuel behavior could pave the way for enhanced reactor performance, improved safety protocols, and increased public confidence in nuclear power. As Generation IV nuclear reactors like the TerraPower NatriumTM strive to redefine the standards for efficiency and environmental sustainability, insights gained from this study may play a pivotal role in shaping the regulatory frameworks and technological standards of the future. 

The TerraPower NatriumTM uses a sodium-cooled fast reactor (SFR) as a heat source with metallic uranium-zirconium (U-Zr) as its fuel. While this design draws on the extensive experience gained from previous SFR projects at DOE national laboratories such as INL, there are still uncertainties about how the U-Zr metallic fuel will behave during reactor operation. 

Transitioning from the discussion of U-Zr metallic fuels, which offer enhanced stability, efficiency, and accident tolerance compared to traditional uranium oxide (UO2) fuel, it is crucial to address the challenges associated with these advancements. Despite their benefits, concerns arise, particularly in terms of potential fuel swelling and compatibility issues with coolant materials. 

Amidst these challenges, Generation IV nuclear reactors emerge as a promising solution, marking a significant leap toward cleaner and more sustainable power generation. These reactors are not only more efficient but also environmentally friendly when compared to conventional reactors or fossil fuel-based power plants. 

“Historically, SFRs utilize a closed fuel cycle, and the metallic fuel slugs used were designed with a large sodium-bonded gap to accommodate swelling,” Dr. Tonks said. “However, these new commercial reactor designs may require an open fuel cycle, which precludes the use of a sodium bond. But when you change the fuel slug geometry, you can introduce room for swelling.” 

A closed fuel cycle involves the recycling of used nuclear fuel and extracting reusable materials to minimize waste and enhance resource utilization, while an open fuel cycle uses nuclear fuel only once, generating more waste with less reprocessing, and storing the entire spent fuel without recycling. Drs. Tonks and Aitkaliyeva are considering a slightly different fuel slug model than has been used in the past. 

Known as an annular or ring-shaped design, it’s essentially a cylinder with a hole down its center. This configuration avoids the need for liquid sodium surrounding the fuel but has also been shown to change how the fuel microstructure evolves during reactor operation. While the reason for this behavior shift is unknown, the research team has some theories. 

“We propose that Zr redistribution and swelling contribute to the shrinking of the center annulus and that the high porosity in the outer region results in low thermal conductivity. Those conditions cause higher fuel temperatures that, in turn, create more Zr redistribution and swelling,” Dr. Aitkaliyeva said. “We will test our hypothesis using structure and property data from annular fuel and multiscale modeling and simulation using the BISON and MARMOT fuel performance codes developed by Dr. Tonks.” 

If they can determine what’s triggering the change, it could be a significant step in the future of nuclear energy. 

“We are excited for the chance to investigate the novel behavior that occurs in annular metallic fuel and help enable advanced reactors for carbon-free power generation in the future,” Dr. Tonks said.