2018 | AFC ACCOMPLISHMENTS 64 The Grain-size Effect on Thermal Conductivity of Uranium Dioxide Principal Investigator: Krzysztof Gofryk (INL) Collaborators: Jie Lien (RPI), Michael Tonks (UF) Figure 1. (a) XRD spectra show that the sintered pellets have UO2+x structure with `x' values calculated by peak positions as shown in (b) for the high angle section. Superimposing feature of Raman spectra (c) indicates similar degree of interaction between defects and UO2 crystal structure in the sintered pellets. Uranium dioxide is one of the most studied nuclear materials as it is used as the primary fuel in commercial nuclear reactors.There are around 500 active nuclear reactors, producing more than 15% of the total electricity worldwide. In a reactor, the heat energy produced from the nuclear fission events inside the fuel pellets is transformed into electricity.Thus, the heat transport mechanism, i.e., thermal conductivity of the fuel material is an important parameter for fuel performance, regarding its efficiency and safety. Project Description: A nuclear reactor operates at extreme environments that can include high temperature, high pressure, and high irradiation. As a result, a fuel pellet undergoes severe structural changes under irradiation conditions, including grain subdivision, fission gas bubbles growth and redistribution and extended defects accumulations.