The Fukushima Daiichi Nuclear Power Plant lost its core cooling function due to the massive tsunami generated by the 2011 off the Pacific coast of Tohoku Earthquake, which caused core meltdown, resulting in high temperature inside the containment vessel and exposing the RPV pedestal, a reinforced concrete structure, to an unusually high temperature environment. After the earthquake, water was poured into the containment vessel to cool the molten core, and the concrete structure was gradually cooled in the process. Since it will take at least 40 years to remove the fuel from the core, the integrity of the RPV pedestal is a major concern for the decommissioning of the Fukushima Daiichi Nuclear Power Plant. In order to assess the long-term integrity of the RPV pedestal, a horizontal loading test was conducted using a 1/6 scaled model of the RPV pedestal of Unit 1 considering the effect of the high temperature heating and subsequent wet conditions. And then, the static stress analysis of the RPV pedestal was performed considering the degradation phenomena revealed by the experiments. As a result, it was confirmed that the RPV pedestal of Unit 1 would be structurally sound for 40 years against the current design basis earthquake even if the material degradation due to severe accident and aging was considered.
In this paper, several modified stainless steels (SSs) with different contents of Ta (0.13-0.61%) and C (0.010-0.046%) were produced to measure the electrochemical potentiokinetic reactivation (EPR) ratio and investigate their crevice corrosion resistance, stress corrosion cracking (SCC) susceptibility and crack growth rate (CGR) in a simulated boiling water reactor (BWR) environment. As a result of the EPR tests, we found that the Ta/C ratio must be ≥ 13 to suppress sensitization by stabilization heat treatment. If the Ta/C ratio is ≥ 19, sensitization can be suppressed without stabilization heat treatment. In the crevice corrosion test under γ-ray irradiation, the maximum corrosion depth in the Ta-modified SSs was smaller than that in type 316L SS. Ta-modified SSs had better crevice corrosion resistance than 316L SS. In the creviced bent beam test, there was no SCC in any of the Ta-modified SSs, whereas cracks were found in four of seven specimens of 316L SS. The CGR test was conducted using 0.5 T-compact tension specimens. Crack growth rates of the Ta-modified SSs were lower than that of 316L SS. The crevice corrosion resistance and SCC resistance were improved by Ta addition. We assumed that Ta addition can improve the repassivation response.
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