Deposition of Cobalt-60 on the Stainless-Steel Surface Used for the Primary Cooling System of a Boiling Water Reactor

The characterization of radioactive corrosion products deposited on the internal surfaces of recirculation pipes was carried out to determine the deposited amounts and configurations. The test specimens were taken from the auxiliary pipes of the recirculation lines of Shimane Nuclear Power Plant after operation for ∼20 000 effective full-power hours. As a result of characterizing them, it was demonstrated that ⁶⁰Co, which determined the contact dose rate, was difficult to remove by mechanical washing the amount of ⁶⁰Co deposited was reduced by more than a factor of 10 by oxalic acid polishing and by a factor of ∼1000 by electro-polishing both the particle size and ⁶⁰Co specific activity of the deposits removed by the mechanical washing (designated as soft deposits) were the same as those of the crud suspended in the reactor water the ⁶⁰Co specific activity of the deposits removed by the oxalic acid polishing (designated as hard deposits) was ten times greater than that of the suspended crud. It was inferred that the soft deposits were formed by direct deposition of the suspended crud, and the hard deposits were formed by oxidation of the stainless-steel base metal with ⁶⁰Co being taken into the oxide layers.