Our vision is that a first-principles understanding of electron and phonon transport addressing the complexity of irradiation-induced defects in nuclear fuels will provide the necessary tools to enhance thermal transport by tailoring defects and microstructure. Our vision will be examined from the perspective of two thrust areas. The first thrust will tackle phonon transport in advanced oxide fuels, and the second thrust will emphasize electron transport in advanced metallic fuels. Both energy transport phenomena contain rich physics that are not well understood and can be investigated using simple systems. 

The model fuel for the phonon transport thrust will be single crystal and polycrystalline thorium oxide (ThO2) and ThO2 doped with uranium. ThO2 is an actinide-bearing material that does not contain 5f electrons and will serve as a baseline that enables the investigation of phonon transport mechanisms in defective fuel without the complication of strong spin-orbit coupling of 5f states. Doping with uranium, while dictated by the technology of the thorium fuel cycle, is a natural way to introduce the effects of 5f electrons, such as the presence of multiple oxidation states, and coupling to magnetic and structural degrees of freedom in a controlled fashion. 

The model fuel for electron-mediated thermal transport will be the binary uranium-zirconium (UZr) system. For Zr concentrations of interest to the nuclear energy community, this system has two primary low-temperature phases: the orthorhombic α phase and the hexagonal δ phase. The choice of these model systems introduce contrasting and intriguing aspects of 5f electrons. The uranium atoms in doped ThO2 will be separated from each other, and their impact on the physical and chemical properties of the lattice will fundamentally differ from the case of uranium-based fuel, where the 5f electrons hybridize into electronic bands. 

This challenge is organized around a carefully constructed set of science questions that seek to close key knowledge gaps related to thermal energy transport in 5f-electron materials. The phonon and electron transport thrusts cross cuts the science questions and provides vertical integration for the research within TETI.