Development of a suppression method for deposition of radioactive cobalt after chemical decontamination: (III) the suppression mechanism with preoxidized ferrite film for deposition of radioactive cobalt

Abstract

The Hitachi ferrite coating film process (Hi-F) has been developed to lower recontamination after chemical decontamination. In this process, the chemical decontamination process is carried out, and a fine Fe₃O₄ coating film is formed on the surface of stainless steel piping in an aqueous solution. In order to improve the suppression of ⁶⁰Co deposition further, we combined the original Hi-F with a preoxidation step. We found the deposited amount of ⁶⁰Co with preoxidized Hi-F coating film (OHi-FC) was 1/10 of that for non-coated specimens. In this study, we investigated the suppression mechanism of ⁶⁰Co for the OHi-FC. The composition of OHi-FC was changed from Fe₃O₄ to Fe₂O₃ and then the crystals in the OHi-FC grew three times larger than those of the original Hi-F coating film. Consequently the corrosion amount of the stainless steel base metal was reduced by getting larger grains in the coating film. Because ⁶⁰Co was incorporated into the corrosion oxide, the suppression effect of ⁶⁰Co deposition by preoxidation was attributed to the suppression of the formation of the corrosion oxide by the OHi-FC.

Keywords:

• boiling water reactor
• chemical decontamination
• recontamination
• oxidized film
• stainless steel