⁶⁰Co deposition on carbon-steel structural materials after seawater infiltration in BWR plant

Seawater infiltration occurred during shutdown of the Hamaoka Unit 5 (H-5). Chloride ion (Cl^-) is known to affect the corrosion behavior of carbon steel, and it may change the properties of the oxide film formed on the surface. ⁶⁰Co deposition in high-temperature water is strongly related to the oxide film properties, and any change in the properties may affect the ⁶⁰Co deposition after the plant is restarted. This paper shows the results of ⁶⁰Co deposition tests of carbon steels under simulated H-5 water conditions. Specimens for the ⁶⁰Co deposition tests were prepared in three steps, which simulated the conditions of normal plant operation, seawater infiltration, and chemical decontamination after the infiltration. The first step was a prefilming step under Normal Water Condition (NWC). The second step included two different conditions: seawater infiltration and keeping after infiltration. Prefilmed specimens were immersed in 450 ppm Cl^- diluted artificial seawater at 513 K for 24 hours. Following that, the specimens were immersed in 50 ppm Cl^- diluted artificial seawater at 323 K for 100-500 hours. During the second step, the prefilming oxide (NiFe₂O₄) flaked off in spots. In the third step, the oxide remaining on some specimens after the second step was removed chemically. The three types of prepared specimens, that is, a prefilmed specimen, an exposed specimen, and an oxide-removed specimen, were used for the ⁶⁰Co deposition tests using 0.015 Bq/cm^{3 60}Co solution for 500 or 1000 hours under NWC conditions. After the deposition tests, the ⁶⁰Co activity was measured with a Ge detector. From the results of the deposition test, at the spots where flaking occurred in the second step, only loose hematite was formed, and generation of a new protective film was not observed. The amount of ⁶⁰Co deposited on the exposed specimen was more than that on the prefilmed and oxide-removed specimens. The simulated infiltrating conditions inhibited the regeneration of a protective film and caused an increase in the amount of ⁶⁰Co deposited. (author)