Processing method for radioactive gaseous wastes

A dehumidifier incorporating hollow thread membranes having a large steam permeation coefficient is used for a dehumidifying cooler or a dehumidifying tower for processing off gases from which water content is separated while passing through a hydrogen recombiner. A portion of gases on the primary side dried by the dehumidifier is used for the secondary side of the hollow thread membranes for purging. The steam permeation performance is further improved by sucking the secondary side to a pressure lower than the primary side by a vacuum pressure of a main condenser or using a purging vacuum pump. The humidity of the off gases is determined, for example, to 40% or less. When the dryness of the gas on the primary side dried by using the dehumidifier such as upon startup of the system is low, dried gas in the plant is used, as the purging gas, instead of the gases on the primary side dried by the dehumidifier. Then, the moisture in the off gases can be removed continuously to make the dehumidifying cooler and refrigerators unnecessary thereby enabling to save control for the inner side of the plant, facilitate the maintenance and shorten the term for the maintenance to about 1/4. (N.H.)