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 363 K

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%.