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Development of a Suppression Method for Deposition of Radioactive Cobalt after Chemical Decontamination: (II) Consideration of Fe3O4 Plating Mechanism on Stainless Steel in Aqueous Solution at 363K

Recently, chemical decontamination at the beginning of periodical inspection has been applied to many Japanese boiling water reactors in order to reduce radiation exposure. However, following the chemical decontamination, a dose rate increase can be seen in some plants after just a few operation cycles. The Hitachi ferrite coating (Hi-F Coat) process has been developed to reduce the recontamination by radioactive cobalt after the chemical decontamination. In this process, a fine Fe3O4 coating film is formed on the stainless steel base metal of the piping following the chemical decontamination in aqueous solution at 363 K. In this study, we investigated a Fe3O4 plating mechanism on the base metal in aqueous solution at 363K by measurements using a quartz crystal microbalance. We found that the Fe3O4 film grew in three steps. First, the Fe3O4 particles were produced on a stainless steel surface. Second, the Fe3O4 particles grew as dome shapes and the converged domes became filmlike. Third, the film grew and became a closely packed Fe3O4 film. Furthermore, we determined the equation of the time dependence of the Fe3O4 film amount using crystal growth theory. The equation predicted the film amount at 10,000 s within a margin of error of 5%.