Investigation of Cobalt Deposition Behavior with Zinc Injection on Stainless Steel under BWR Conditions

Abstract

Radioactive58Co and 65Zn were used together to investigate the deposition behavior of cobalt and zinc in the oxide film formed on the stainless steel piping of an apparatus simulating the high temperature and pressure conditions of a boiling water reactor. Using these radioactive tracers allowed the experimental setup to closely approximate the conditions found in actual plants. The accumulation of 58Co and 65Zn on the stainless steel piping was monitored using an online gamma-ray detector. The results were as follows. At higher zinc concentrations, the early period of fast deposition was shortened and the later deposition phase was slowed. Two mechanisms appeared to be responsible for the suppression of cobalt deposition by zinc injection. The first worked by decreasing the growth rate of the oxide film. This effect appeared most conspicuously in the outer oxide layer under normal water chemistry conditions. The second mechanism worked by reducing the cobalt concentration in the oxide film. This second effect was prominent in the inner oxide layer under hydrogen water chemistry conditions.