Influence of Internal Thermal Radiation on Heat Transfer in UO₂ Fuel Elements

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.