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Mission Statement: To accurately predict, from first principles, thermal energy transport in actinide materials in extreme environments. In nuclear fuel, irradiation-induced defects effectively scatter thermal energy carriers (electrons and phonons), greatly reducing the capacity of the fuel to transport heat to the coolant for eventual electricity generation. For example, in oxide fuels, the thermal conductivity decreases by as much as 70% over the operational lifetime of the fuel. This reduction significantly impacts fuel performance, safety margins, and the amount of usable energy. However, in some special cases, microstructure evolution can lead to local increases in thermal conductivity. For oxide fuels, examples include a reduction in phonon scattering associated with the transformation of faulted loops to perfect loops (a change in the strain field), transformation of a loop ensemble into a line segment (change in defect dimensionality), and defect segregation at interfaces (cooperative effects). Indeed, the myriad of defect types and interactions in nuclear fuel under irradiation naturally leads to the supposition that the deleterious losses in thermal conductivity can be mitigated by controlling defect evolution. The Center thus adopts the vision that a first-principles understanding of electron and phonon transport addressing the complexity of irradiation-induced defects will provide the necessary tools to control thermal transport in nuclear fuel. Our vision will be examined from the perspective of two thrusts. The first will tackle phonon mediated thermal transport in advanced oxide fuels (thorium oxide - ThO2 and thorium/uranium mixed oxide Th1-xUxO2). The second thrust will emphasize electron and phonon mediated thermal transport in advanced nitride fuels (uranium nitride - UN and thorium nitride - ThN). Both thermal energy transport phenomena contain rich physics that are not well understood and can be investigated using simple systems. To meet our vision we have defined four research goals that represent significant challenges in the field of thermal transport.
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