Effects of Hydrogen Peroxide on Radioactive Cobalt Deposition on Stainless Steel Surface in High Temperature Water

Abstract

In order to understand the effects of hydrogen peroxide on ⁶⁰Co deposition under water chemistry conditions of boiling water reactors, deposition amounts of ⁶⁰Co on the stainless steel specimens were measured by changing hydrogen peroxide (H₂O₂) and oxygen (O₂) concentrations in a high temperature water loop, and correlations between the nature of oxide film on the specimen surface and its deposition were examined by separating the contributions of inner and outer oxide layers to the deposition. The results are summarized as follows.

1.	The weight change of the specimens and the amount of 60Co deposition were strongly affected by the presence of H2O2. Weight gain of the specimens exposed to various H2O2 concentrations after a 200-h pre-exposure to 200 ppb H2O2 was less than 2μg/cm2 for 1,000 h; this was caused by protection effects of thin and dense oxide film.
2.	The amount of 60Co deposition on the specimen exposed to 200 ppb H2O2 was much less than the amounts of those exposed to 10, 5 and 0 ppb H2O2 in the water with 50 ppb H2, but in the case of 200 h-pre-exposure to 200 ppb H2O2, the latter specimens had deposition amounts as low as that of the specimen exposed to 200 ppb H2O2.
3.	The amount of 60Co deposition had a fairly linear correlation with the amount of outer oxide layer, except in the case of specimens exposed to the condition without H2O2 from the first. In the latter case, 60Co was expected to be included in a Cr-rich inner oxide layer.

KEYWORDS:

• cobalt
• BWR type reactors
• oxygen
• hydrogen
• hydrogen peroxide
• electrochemical corrosion potential
• oxide film
• deposition
• corrosion
• normal water chemistry
• hydrogen water chemistry