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Report No.
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Preliminary evaluation of the fuel debris behavior below the RPV lower head boundary of 1F Unit-2

Bando, Yamato*; Sasaki, Ryotaro*; Fukuda, Takanari*; Yamaji, Akifumi*; Yamashita, Takuya  

As one of the three tasks of "Project of Decommissioning, Contaminated Water and Treated Water Management (Development of Analysis and Estimation Technologies for Characterization of Fuel Debris) (Development of Estimation Technologies of RPV Damaged Condition, etc.)", this study presents evaluation of the fuel debris behavior below the damaged RPV lower head boundary of Fukushima Daiichi Nuclear Power Station (1F) Unit-2. The focus of the study is to evaluate the debris behavior at the time of / after the failure of the RPV boundary. It is expected to provide more comprehensive understanding of the precedingly obtained muon image, which seemed to indicate that a large amount of highly-dense materials distributed between the RPV lower head and the thermal insulation structures just below the RPV. The Moving Particle Semi-implicit (MPS) method is being developed to evaluate the fuel debris behavior in/under the actual plant geometry and conditions. The melt behavior analysis code, based on the MPS method, is being developed to analyze the following two debris behaviors. Firstly, the debris discharge behavior from penetration tube structures is analyzed. The solidified debris blocks are represented by rigid bodies, using the Passively Moving Solid (PMS) model with consideration of decay heat of the oxidic fuel debris. The relocations of the oxidic debris involving melting of the surrounding metallic debris and the penetration tube wall structure are analyzed. Secondly, the melt behavior on / through the multi-layered thermal insulation structures below the RPV is analyzed. The discharged melt from the RPV boundary may freeze on the insulation plate, depending on the thermal condition in the pedestal and the discharged melt history.

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