Heat transfer in flat-plate UO2 fuel elements has been studied theoretically. Particular attention has been focused on internal-radiation heat transfer. Local radiant-heat fluxes have been calculated and the relative importance of radiation compared to conduction has been established. The results show that even at elevated temperatures single crystal UO2 is transparent enough, and internal radiation contributes significantly to high-temperature heat transfer. The presence of scattering at the grain boundaries in polycrystalline UO2 increases the extinction coefficient considerably. Because of this, radiant heat transfer was found to be unimportant compared to conduction.