Status of experimental and analytical thermal-hydraulic studies on severe accident events at Fukushima Dai-ichi NPP

Takase, Kazuyuki; Yoshida, Hiroyuki  ; Liu, W.; Misawa, Takeharu; Nagatake, Taku ; Yamashita, Susumu   

With regard to the severe accident in the BWR at the Fukushima Dai-ichi Nuclear Power Plant, in order to clarify a progress of the thermal-hydraulics phenomena in the nuclear pressure vessel and improve the current accident management, experimental and analytical studies were started by Japan Atomic Energy Agency. Three studies are carried out for experimental research. The first is a study of the influence of sea water on reactor core cooling. The second is a study that investigates the melting behavior of the fuel rod using a metal rod and an indirect electric heater. The third is a study that evaluates the heat transfer characteristics of molten debris accumulated on the bottom head of the lower plenum in the pressure vessel. The experimental data are used for verification of the analytical results. For analytical research, the following three studies are conducted. First, the situation in which the heating area of the fuel rods is exposed from the water surface is clarified numerically. Second, the melting and degradation behavior of the fuel assemblies is predicted quantitatively. Third, an analysis method is developed in order to precisely predict the three dimensional relocation distribution of the molten debris on the bottom head in the pressure vessel under the condition that many BWR control rod guide tubes exist.