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Development of numerical simulation method for relocation behavior of molten materials in nuclear reactors; Relocation behavior in a simplified core structures

Yamashita, Susumu ; Takase, Kazuyuki; Yoshida, Hiroyuki 

In accidents of the Fukushima Daiichi Nuclear Power Plants, by stop of the emergency core cooling system, fuel rods were overheated due to the radioactive decay heat and the oxidization of fuel cladding. Although it is inferred that the core degradation occurred, condition inside the core still has not been revealed. Especially, in order to precisely understand the accumulation condition of debris in lower plenum, detailed and phenomenological relocation process of molten fuel is quite important. In this problem, since an experiment is extremely difficult, numerical simulation will be useful tool for investigating conditions in reactor core. However, existing codes can not be phenomenologically treated relocations process. Therefore a phenomenologically-based numerical simulation method for predicting the melting core behavior including solidification and relocation based on the computational fluid dynamics has been developed in JAEA. Last paper, ICONE 23, in order to distinguish a fuel component which possesses a heat source such as a decay heat from structures which does not possess a heat source such as a core plate, control rod guide tubes and so on, we developed three phases and three component multiphase flow simulation code and performed preliminary analysis of molten core relocation behavior using simplified core structures. As a result, we obtained reasonable results. However, we have not carried out validations for the numerical code yet. In this paper, we show that the results of the numerical test for evaluating the validity of the numerical code and also show that the development of the radiation heat transfer model and its preliminary analysis. In addition, we will report the preliminary analysis of the relocation behavior of molten materials in the simplified core support structure.

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