Development of a Suppression Method for Deposition of Radioactive Cobalt after Chemical Decontamination: (I) Effect of the Ferrite Film Coating on Suppression of Cobalt Deposition

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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.