2018 | AFC ACCOMPLISHMENTS 49 The thermal conductivity of UB2 and UB4 has been measured up to 1500˚C and was found to increase at temperatures higher than 1000˚C, thus enhancing the accident tolerance classification of these materials. microstructures, taken in backscatter mode, from a polished (a) UB2 and (b) UB4 sample are shown.The UB2 sample (95%TD) is phase pure with some intergranular porosity and average grain size of 7.5 μm.The UB4 sample (93%TD) shows more exten- sive porosity, as expected, and signs of a small fraction of a secondary phase (bright phase). Since the XRD results indicate a phase pure material, this secondary phase is expected to be less than 5%.The average grain size for this sample is 4 μm. The thermal conductivity of UB2 and UB4 has been calculated up to 1500˚C using experimental data for the thermal expansion and thermal diffusivity, and literature values for specific heat. It is shown in Figure 4(a) as a function of temperature along with literature data for UO2 and the conventionally sintered UB2 pellet (White, 2015). The calculated conductivity for UB2 is significantly higher than that of UB4 at room temperature and at higher temperatures, past 1000˚C. The conductivity reported for the conventionally sintered pellet, although it follows the same trend as that for the SPS sample measured in this study, it is significantly lower over the entire range of measured temperature. At temperatures past 1000˚C the thermal conductivity increases for both UB2 and UB4, meaning at these temperatures the thermal gradients would be minimized thus making them good candidates for accident tolerant fuel. In Figure 4(b) the ratio of thermal conductivity for UB2 and UB4 over that of UO2 is shown to better illustrate this point. For the case of UB2, the conductivity is almost 12 times higher than of UO2 at 1500˚C whereas for UB4 it is 7 times higher.The increase in the thermal conductivity for these materials is expected due to the increased electronic contributions resulting from their metallic nature. The thermal conductivity trend for UB2 reveals that phonon contributions are significant in the lower temperatures and as the temperature increases the electronic contribution becomes prevalent while in the case of UB4, the electronic contribution seems to be prevalent throughout the studied temperature range. Figure 4: (a) Thermal conductivity of UB2 and UB4 samples sintered by SPS at 1750˚C compared to conventionally sintered UB2 and UO2 , and (b) Ratio of thermal conductivity of UB2 and UB4 to UO2 .