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

Abstract

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.