Hydrous pyrolysis using superheated steam was demonstrated for safe storage of a radioactive-Cs-adsorbed adsorbent based on Prussian blue. Iron oxidation and CN decomposition in the adsorbent were observed at 500 degrees C. After hydrous pyrolysis, almost all the adsorbed Cs remained in the decomposition products. Results show that this method effectively avoids thermal runaway caused by exothermic reactions like iron oxide formation.