Development of a suppression method for deposition of radioactive cobalt after chemical decontamination. (3) The suppression mechanism with preoxidized ferrite film for deposition of radioactive cobalt

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. (author)