Hydrocarbons are removed from a waste gas including oxygen, hydrocarbons and radioactive Krypton-85 (hereinafter referred to as "Kr-85"), which is generated in a nuclear reactor and a fuel recycling plant without adopting the reduction method of oxygen by adding hydrogen; the resultant gas is caused to pass through an activated charcoal layer and Kr-85 is adsorbed to the activated charcoal layer; the rest of gas is discharged to the atmosphere; further oxygen remaining in the activated charcoal layer is returned to a process of removing hydrocarbons; and thereafter Kr-85 adsorbed to the activated charcoal layer is desorbed, condensed and recovered.
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