Influence evaluation of corrosion on long-term integrity of spent fuel assembly component materials exposed to unusual corrosive environment at unit 1-4 spent fuel pools of Fukushima Dai-ichi Nuclear Power Stations

In the Fukushima Dai-ichi Nuclear Power Station (1F-NPS) accident, the spent fuel pools (SFPs) had lost their functions of decay heat removal and water supply due to the station blackout. Since fresh and sea waters were injected into and concrete fragments by hydrogen explosions fell into the SFPs, the fuel assemblies (FAs) have been exposed to much more corrosive environments than usual ones. It is planned to transport the FAs in SFPs of Units 1-4 to a common pool set up on the 1F-NPS site where they will be stored for a long time. Therefore, it is necessary to comprehend the long-term influence of corrosion on integrity of the FAs during wet storage. In this study, a screening study on corrosion phenomena and a preliminary investigation for an evaluation method on long-term integrity of FAs experienced unusual corrosive environment were carried out in views of fundamental features of corrosion. The screening study have led to the following two features of FAs from the viewpoint of the integrity as important phenomena to be further investigated; 'fission product confinement of cladding tube' and 'structural integrity of FA'. In terms of 'fission product confinement of cladding tube', it was shown experimentally that influence of the exposure to an unusual corrosive environment was low. On the other hand, in terms of 'structural integrity of FA', a concept of experimental methodology for predicting long-term corrosion behavior was preliminary studied for preferentially selected FA local parts composed of different metals (zircaloy-2 and stainless steel). The basic test utilizing simulated specimens composed of different metals was performed for the preliminary verification of appropriateness of the present experimental methodology. The basic test result indicated that crevice was main corrosion morphology and that the possibility of crevice progress seemed to be high with increasing test temperature in the simulated specimen which was composed of stainless steel and zirconium alloy in the case of the exposure to the condition of 90degC and 10,000ppm Cl^- for 100 hours. (author)