Development of numerical simulation method for relocation behavior of molten materials in nuclear reactors; Analysis of relocation behavior for molten materials with a simulated decay heat model

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 because fuels, control rods, and other components in a reactor vessel was melted and relocated, 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. In order to correctly estimate progress of the relocation phenomena in the reactor core, a numerical simulation code that can phenomenologically evaluate the melting phenomena is required. 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 21, we carried out the calculation of relocation behavior using three phase (solid/liquid/gas) and two components (metal and gas) fluid flow simulation model, however, there is only one component for metal. Therefore, the model cannot distinguish fuel material with decay heat from core internal materials. In this paper, we show the brief overview about the extended code, which is added one more component to the previous code to distinguish a fuel material with constant heat source simulating decay heat in the energy equation from core internals, and also show that the numerical results of relocation behavior for molten fuel and core internals in a reactor core.