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Creep damage evaluations for BWR lower head in severe accident

重大事故下におけるBWR下部ヘッドのクリープ損傷評価

勝山 仁哉; 山口 義仁; 根本 義之; 加治 芳行; 逢坂 正彦

Katsuyama, Jinya; Yamaguchi, Yoshihito; Nemoto, Yoshiyuki; Kaji, Yoshiyuki; Osaka, Masahiko

東京電力福島第一原子力発電所のような沸騰水型原子炉(BWR)のRPV下部ヘッドは、形状が複雑で多数の制御棒案内管が存在するため、その破損挙動は複雑である。そこで我々は、重大事故時のBWR下部ヘッド破損について、クリープ損傷機構を考慮した熱流動構造連成解析に基づく評価手法を整備した。本研究では、事故シナリオの違いを想定し、溶融デブリの深さや発熱位置の違いが破損位置に及ぼす影響について評価した。その結果、BWR下部ヘッドの破損やデブリの流出は、貫通部における制御棒案内管やスタブ管で生じることを示した。

It is difficult to assess rupture behavior of the lower head of reactor pressure vessel in boiling-water-type nuclear power plants due to severe accident like Fukushima Daiichi because Boiling Water Reactor (BWR) lower heads have geometrically complicated structure with a lot of penetrations. Therefore, we have been developing an analysis method to predict time and location of RPV lower head rupture of BWRs considering creep damage mechanisms based on coupled analysis of three-dimensional Thermal-Hydraulics (TH) and thermal-elastic-plastic-creep analyses. In this study, we performed creep damage evaluations to investigate the effects of the debris depth and heat generation locations on failure behavior of lower head. From the analysis results, we discussed the outflow paths of the relocated molten core to the containment, and it was concluded that failure regions of BWR lower head are only the control rod guide tubes or stub tubes under simulated conditions.

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