[en] In the last decade, chemical decontamination at the beginning of periodical inspection has been applied to many Japanese BWR plants in order to reduce radiation exposure. However, following the chemical decontamination, a rapid dose rate increase can be seen in some plants after just a few operation cycles. Oxide film, which easily incorporates radioactivity, might be formed after the chemical decontamination. We developed a new way to reduce the recontamination after the chemical decontamination to maintain long-term continued decontamination effects without any chemical injections or chemical controls in reactor water during operation. In our approach, a fine ferrite film is formed by the Hitachi Ferrite Coat process after oxide films formed during the plant operation are removed by the chemical decontamination process. We select Fe(HCOO)₂ aqueous solution, H₂O₂, and N₂H₄ as the treatment chemicals for fine ferrite film formation for suitable BWR plant application. Our laboratory experiment results confirm a ⁶⁰Co deposition reduction effect of 1/5 compared with that of nontreatment for up to 3,100 hours. The fine ferrite film that was formed on the specimen before the ⁶⁰Co deposition test remains as a film structure after the test. The corrosion amount of the specimen is suppressed to 1/4 through the effect of the fine ferrite film. (author)