2018 | AFC ACCOMPLISHMENTS 65 Thermal properties of the fuel material are greatly affected by these changes which ultimately affect the performance of a reactor. Numerous theoretical and experimental studies have been carried out to understand how these microstructure changes affect thermal transport properties of UO2. In an oxide fuel, the lattice vibrations (phonons) responsible for the heat transport are scattered by different scattering centers, such as defects, grain boundaries, and phonon-phonon interactions. Depending upon the temperature range, different scattering mechanisms dominate at different temperature regimes. For instance, umklapp phonon- phonon scattering dominates the thermal conductivity at high- temperature, while the point-defect and boundary scattering govern the heat transport at intermediate and low temperatures, respectively. At low temperatures where the phonon mean free path is comparable to the grain-size, the grain boundary scattering mechanism is the main factor limiting the thermal conductivity. Here we have investigated the grain boundary scattering effect on the thermal transport behavior of uranium dioxide having different grain- sizes. We analyzed the results obtained using molecular dynamics (MD) calculations. Accomplishments: We have carried out systematic studies on the grain-size effect on thermal conductivity of UO2 by performing measurements at low temperatures to study different scattering mechanisms, focusing on grain boundary scattering.The We unveiled (experimentally and theoretically) details of grain boundary scattering and its impact on the thermal conductivity in uranium dioxide.