Investigation of cobalt buildup behavior and suppression by zinc injection on stainless steel under HWC conditions using simultaneous continuous measurements of corrosion and cobalt buildup

In boiling water reactor (BWR) plants, cobalt-60 (⁶⁰Co) is the main source of radiation exposure, and it builds up on oxide films of structural materials. The ⁶⁰Co buildup is caused by its incorporation into the oxide films. In the BWR plants using hydrogen water chemistry (HWC) to mitigate the oxidative environment, Zn injection has been applied to reduce the ⁶⁰Co incorporation. In this work, we studied the incorporation mechanism of ⁶⁰Co into the oxide films on type 316 stainless steel and the suppression mechanism of ⁶⁰Co incorporation. In order to discriminate between coprecipitation and adsorption of ⁶⁰Co incorporation under HWC conditions, we measured the corrosion amount of the base metal and the ⁶⁰Co buildup amount, using simultaneous continuous measurements for 500 h. The ⁶⁰Co incorporation increased with time both with and without Zn injections. We found that the time dependencies of ⁶⁰Co incorporation with and without Zn have one and two regions, respectively. In the initial stage for both, ⁶⁰Co was incorporated mainly by coprecipitation. After 100 h without Zn, ⁶⁰Co was incorporated by both coprecipitation and adsorption. These results mean that Zn suppressed both coprecipitation and adsorption of ⁶⁰Co.