In nuclear fuel, there’s a heat-related sweet spot that not only ensures the reactor operates safely, but also that it generates as much energy as it can.
“Heat transport is critical for both reactor efficiency and safety. It determines how fast the thermal energy generated from nuclear reaction can be harvested to generate electricity,” says Nuclear Engineering and Engineering Physics Assistant Professor Yongfeng Zhang. “In addition, if heat is not transported out efficiently, the temperature inside of the fuel can get too high and potentially cause safety issues.”
Yongfeng Zhang
In ceramic nuclear fuels, which most current nuclear reactors use, heat transport is mediated by phonons, which are units of vibrational energy that arise from atomic oscillations within a crystal. Disruptions, or lattice defects, in the orderly arrangement of atoms in crystalline solids can hinder phonon transport. Unfortunately, the intense irradiation inside nuclear fuels is perfect for creating lattice defects that hinder heat transport.
“We can picture the phonons as a wave of water in a lake; objects like rocks or floating wood can deflect and slow down the wave. The nature, size and number density of such objects all affect their ability of slowing down the wave,” Zhang says. “In a similar way, the efficiency of heat transport in nuclear fuels will be dependent on the nature, size and number density of lattice defects that are generated by irradiation.”
As part of the DOE Office of Science Energy Frontier Research Center for Thermal Energy Transport under Irradiation (TETI) led by Idaho National Laboratory, Zhang is a member of an internationally recognized multi-institutional team of experimentalists and computational materials theorists aiming to develop a comprehensive, atom-to-mesoscale understanding of how lattice defects affect phonon and electron transport in advanced nuclear fuels.
Zhang is studying how atomic-level defects form, move and grow in nuclear fuels such as uranium dioxide, thorium dioxide and uranium nitride. Specifically, he’s working to characterize the size and distribution of sub-nanometer defects, which are too small to view clearly, even with highly advanced transmission electron microscopes. Instead, Zhang is developing atomistic models of the materials under irradiation. The models will allow him to simulate how tiny defects form and evolve into larger defects whose structures are better known.
“The defects are troublemakers for nuclear fuel performance,” Zhang says, “but on the other hand very interesting to study.”
Irradiation-induced defects can cause the thermal conductivity of oxide fuels to decrease by as much as 70% over the operational lifetime of the fuel. They also limit the service lifetime of nuclear fuels. This reduction significantly impacts fuel performance, safety margins and the amount of usable energy the reactor generates
In oxides, lattice defects usually carry electric charges, either positive or negative, making them different from their counterparts in metals, on which most of our current understanding of lattice defects are established. Zhang and his collaborators are studying how charge affects the structure, movement and interaction of atomic-level defects and their growth into larger ones. Those with opposite signs of charges—a positive and a negative—attract to each other, while two defects with the same signs of charge repel each other, which is why it’s important to take the charges into consideration when analyzing defects in these materials.
Zhang says he’s excited to work with the TETI interdisciplinary team to address the big question of how irradiation affects heat transport in oxide fuels. “The fundamental scientific discoveries that come out of this research center could help us better assess how a fuel’s properties will degrade over time, and more importantly, design new nuclear fuels that are safer, more efficient and can last longer in reactors.”
Featured image caption: Ceramic nuclear fuel pellets are stacked vertically in long metal tubes to power commercial nuclear reactors. Image credit: U.S. Nuclear Regulatory Commission.