Effect of borate addition to fresh water on the self-passivation behavior of carbon steel

This research was conducted with the objective of developing knowledge on the integrity evaluation of carbon steel in the primary containment vessel at the time of taking out fuel debris at the Fukushima Daiichi Nuclear Power Station (1F) of TEPCO Holdings Corporation. The effect of borate administration on the self-passivation behavior of carbon steel was evaluated based on the time course of the national corrosion potential in the natural immersion test, and the analysis results of water before and after the test. From the results, a method to estimate the self-passivation tendency by the carbon steel self-passivation index (SPI) SPI=(0.5[SO₄^2-]+[Cl^-])/(HCO₃^-]+9[B(OH)₄^-]) was presented. Here, [B(OH)₄^-]_E was obtained as B(OH)₄^--equivalent by the following formula. [B(OH)₄^-]_E=[M-Alk]-[HCO₃^-]. That is, the self-passivation tendency of carbon steel in a reactor containment vessel in a borate-containing solution environment may be estimated by sodium pentaborate dosage concentration, M-alkalinity, sulfate ion concentration, chloride ion concentration and bicarbonate ion concentration. According to this technique, when the SPI is less than 0.282, carbon steel self-passivates. (author)