Development of a Suppression Method for Deposition of Radioactive Cobalt After Chemical Decontamination: Application of a Ferrite Film Formation Process to an Actual BWR Plan

摘要

The formation process of a ferrite oxide film (which can effectively suppress radioactive nuclide deposition on piping surfaces) was evaluated from the viewpoints of forming optimum film structures and reducing waste disposal. Both pH and oxidation-reduction potential (ORP) of ferrite film formation solution were found to be important, and the film formation process could be understood on the basis of a Pourbaix diagram of the iron-water system. To make a thin and closely packed oxide film, the pH and ORP values should be maintained within the magnetite stability domain by controlling the hydrazine concentration, which promotes the film formation reactions. Use of chemical solutions such as formic acid and hydrazine was confirmed to get catalyst decomposition into easily handled substances. This film formation process could be evaluated taking into consideration the charge balance and chemical equilibrium equations of each reaction involved in the film formation. It was clarified that preoxidation of the ferrite film under certain oxidizing water chemistry conditions (such as normal water chemistry) in boiling water reactors could further improve the film cobalt deposition suppression performance due to the formation of hematite. Our selected film forming process and waste solution decomposition conditions were confirmed using the simulated flow system apparatus of one-tenth actual plant scale. The method was applied to the actual plant just after the chemical decontamination. After one cycle elapsed, dose rate of the reactor recirculation system piping coated with ferrite film was half that before the ferrite film was formed.