2018 | AFC ACCOMPLISHMENTS 66 polycrystalline samples having different grain-sizes (0.125, 1.8, and 7.2 um) have been prepared by spark plasma sintering technique at Rensselaer Polytechnic Institute(RPI) by Professor Jie Lien research team. After the synthesis several characterization techniques have been employed such as by x-ray powder diffraction (XRD), scanning electron microscope (SEM), and Raman spectroscopy (see Figure 1) to ensure sample quality with desired characteristics. All pellets were fully densified with measured density higher than 95 % TD. The XRD spectrum in Figures 1a and b show that the sintered pellets are single phase UO2. Microstructure characterization was conducted by SEM microscopy and the average grain sizes were estimated to be 0.125, 1.8, and 7.2 um. Furthermore, the Raman spectroscopy (Figure 1c) shows that Figure 2. (a) The low temperature thermal conductivity of UO2 samples. The solid lines represent the least- square fits of the Callaway model to the experimental data; The grain- size dependence (obtained from the Callaway model) of the low- temperature thermal conductivity peak (b), parameters B (c), D (d), and the mean free path of phonons L (e). The corresponding values for the UO2 single-crystal are displayed as a horizontal line in the relevant graphs. The dashed lines are guides to the eye.