Effect of water vapor on tritium permeation behavior in the blanket system

► Tritium bred in the blanket must be effectively recovered to establish the fuel cycle of a fusion reactor. ► We found that coexistence of the amount of water vapor in the purge gas can decrease the tritium recovering ratio. ► Almost no decrease in permeation rate of F82H in the breeding part is observed when water vapor exists in the purge gas. ► Permeability of hydrogen isotopes through Pd–Ag in the recovery part gradually decreases when water vapor exists in the purge gas. It is found that most hydrogen supplied to the purge gas changed to water vapor due to the water formation reaction in the early stage of the blanket operation and that physical or chemical adsorbed water is released in the high concentration into the blanket purge gas when the blanket temperature becomes higher than several hundreds of degrees K if the pre-treatment is not applied to the solid breeder materials. Effect of coexistence of water vapor in the purge gas on permeation behavior of hydrogen through F82H ferritic steel in the breeding part and palladium–silver (Pd–Ag) in the recovery part is discussed because use of them is generally considered for recovery of bred tritium from the solid blanket. Almost no decrease in permeation rate of F82H is observed in this study when water vapor exists in the blanket purge gas. The permeability of hydrogen isotopes through the Pd–Ag pipe gradually decreases when water vapor exists in the blanket purge gas. Properties required in estimation of the hydrogen permeated to the purge gas are experimentally obtained in this study.