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Development of failure evaluation method for BWR Lower head in severe accident; Creep damage evaluation based on thermal-hydraulics and structural analyses

Katsuyama, Jinya ; Yamaguchi, Yoshihito ; Nemoto, Yoshiyuki  ; Kaji, Yoshiyuki ; Yoshida, Hiroyuki 

It is difficult to assess rupture behavior of the lower head of RPV in boiling water reactors (BWRs) due to severe accident like Fukushima Daiichi. This is because BWRs have geometrically complicated structure with a lot of penetrations, and BWR lower head is composed of various types of materials. We have developed an analysis method to predict time and location of BWRs lower head rupture considering creep damage mechanisms based on coupled analysis of three-dimensional thermal-hydraulics (TH) and thermal-elastic-plastic-creep analyses. The detailed three-dimensional model of RPV lower head with control rod guide tubes, stub tubes, and welds are constructed. TH analysis is performed to obtain temperature distribution in relocated debris. Using TH analysis results, structural analysis is carried out to evaluate creep damage distributions using damage criterions. Creep damage evaluation models based on Kachanov and LMP criteria are made. From comparison of damage criterions, it is shown that failure regions of BWR lower head are only penetrations under simulated conditions, although there is a large difference in failure time.

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