2018 | AFC ACCOMPLISHMENTS 67 the chemical bonding in those three- different grain-sized samples are very similar, indicating comparable localized defect interaction with the crystal structure of UO2.The thermal transport properties (the thermal conductivity and thermoelectric power) have been measured in the temperature range 2-300 K and the results were analyzed in terms of various physical parameters contributing to the thermal conductivity in these materials in relation to grain-size.These studies have by performed at Idaho National Laboratory (INL) by Dr. Gofryk’s team. We show that the grain boundary scattering parameters vary systematically with the grain- size below 30 K (Figure 2a). Such a behavior is not observed at higher temperatures where other scattering processes start to dominate. At room temperature the thermal conductivity values of the UO2 polycrystalline samples vary slightly with grain-size (not shown here), and all values are within 5% error of the measurement (and also ~10% smaller than that of the single-crystal).The variation of thermopower (not shown here) suggests that very small oxygen off-stoichiometry might be present and play a role in lowering of the thermal conductivity in the UO2 samples, especially at high temperatures.To evaluate this important observation in more details, single crystals studies (on UO2 crystals with different oxygen content) are required.The thermal conductivity data were analyzed and the variation of different parameters such as grain boundary (B), defects (D), or phonon mean-free path (L) with the grain-size are presented in Figures 2b-e. In order to determine if changes in the measured thermal conductivity at high temperatures are due to the grain boundary Kapitza resistance, Professor Michael Tonk’s team (University of Florida) investigated this behavior using the analytical Kapitza resistance equation.The results obtained indicate the importance of low temperature measurements to study grain boundary scattering in UO2. At higher temperatures other scattering processes are involved in the heat transport in these materials that makes this analysis difficult. In addition, these studies point to the importance of oxygen off-stoichiometry for the thermal conductivity of UO2.