Cobalt Radioactivity Behaviors in a BWR Environment and Countermeasures for Dose Rate Reduction

AbstractWhile under normal water chemistry without any specific metal ions in reactor coolant a high electrochemical corrosion potential caused by highly oxidizing species such as hydrogen peroxide promotes the formation of hematite film on piping surfaces with a densely packed film structure, the presence of a certain amount of nickel ions prevents the magnetite film from changing to hematite by forming a nickel ferrite. This formation of nickel ferrite instead of hematite accelerates cobalt buildup, and this is especially notable for carbon steel. The observed reduction of radioactivity concentration in reactor water by zinc injection or by nickel/iron ratio control can be explained by the role of zinc or nickel in preventing the film on the fuel rod surfaces from changing to hematite, thereby stabilizing the cobalt activity on this surface. A thermodynamic evaluation suggests that zinc ferrite is more stable than cobalt ferrite only when the ratio of cobalt to zinc divalent ions, [Co2+]/[Zn2+], is <0.0...