Evaluation of long-term integrity of the fuel assembly removed from the spent fuel pool, 3; Investigation of chloride ion transport phenomenon across the screw-gap between Zircaloy upper plug and stainless nut immersed in dilute artificial seawater containing Cl ion

Sekio, Yoshihiro  ; Maeda, Koji ; Yamagata, Ichiro ; Ueno, Fumiyoshi  

The upper plug of a fuel rod used for the spent fuel pool (SFP) of Unit 4 in Fukushima Daiichi Nuclear Power Plant (1F) consists of Zircaloy-2 bolt and SUS304L nut, and it forms the screw-gap structure. Therefore, when seawater constituents stays behind in this gap structure by a seawater injection after the accident, crevice corrosion might occurs. This may have an influence on the integrity of a fuel assembly stored in common pool. It is necessary to estimate transport phenomenon of seawater constituents across the screw-gap structure. In this study, the transport phenomenon of seawater constituents was investigated using especially radioactive tracer Cl-36 (Cl) by simulating the immersion condition experienced for the SFP of Unit 4 in 1F. From the results, it was qualitatively and quantitatively confirmed that the Clion was removed from the screw-gap structure by water quality improvement (the decrease of Cl concentration in test solution).