DEVELOPMENT OF A SUPPRESSION METHOD FOR DEPOSITION OF RADIOACTIVE COBALT AFTER CHEMICAL DECONTAMINATION: ENHANCEMENT OF SUPPRESSION PERFORMANCE FOR FERRITE FILM COATING

摘要

For 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. In the chemical decontamination, the oxides that incorporate ~(60)Co are dissolved with reductive and oxidative chemical reagents. Therefore, some base metal of the piping appears on the surface after the decontamination. The oxide film growth rate of the piping during plant operation just after the decontamination is higher than that just before it. In addition, the concentration of radioactivity in reactor water of old plants is higher than that of new plants. Therefore, there is a possibility that the deposition amount of radioactivity on the piping when beginning an operating cycle of a just-decontaminated plant is higher than that just before the decontamination. Our objective was development of a more effective method to reduce the recontamination after the chemical decontamination. In the developed suppression method, called the Hitachi Ferrite Coat (Hi-F Coat) process, a fine ferrite coating film is formed on the base metal of the piping following the chemical decontamination. For the Hi-F Coat process, laboratory experiments confirmed a °Co deposition reduction effect of 1/3 compared to non-coated specimens. The film structure of the ferrite coating was kept after soaking in high temperature water to simulate BWR conditions. The corrosion amount of the ferrite film-coated specimen was suppressed to about half that of a polished specimen. This ferrite film blocked diffusion of oxidants in the reactor water to the base metal and metal ions in the oxide film to the reactor water. However, the ferrite film incorporated a few dissolved °Co ions, because ferrous ions in the ferrite film were exchangeable with ~(60)Co ions in the simulated BWR condition water. In order to prevent the ~(60)Co ion incorporation into the ferrite film, we investigated two procedures to be combined with the Hi-F Coat process. First the prevention effect with pre-oxidation under the normal water chemistry condition for Hi-F Coat process specimens was investigated. These laboratory experiments confirmed a ~(60)Co deposition reduction effect of 1/10 compared to polished specimens. Secondly, the prevention effect of the Co ion incorporation was investigated when zinc ions were injected into the high temperature water. These laboratory experiments confirmed a ~(60)Co deposition reduction effect of 1/6 compared to non-coated specimens without zinc ion injection.