© 2015 The Japan Society of Mechanical Engineers. In the Fukushima Daiichi nuclear power plant accident in 2011, the tsunami attacked the power plant and station blackout occurred. As a result, core meltdown was intended at Unit 1, 2 and 3. To remove molten fuels from the Fukushima Daiichi nuclear power plants, it is necessary to know the current status of the reactors. The current status of the reactors may be affected by the thermal and hydraulic behavior of the coolant in the reactors. In the Fukushima Daiichi nuclear power plant accident, seawater was injected into the reactor to cool down the nuclear fuels. The injection of the seawater may change the thermal-hydraulic characteristics in the reactors due to the changes of the physical properties of the coolant. Therefore, the thermal hydraulic behavior of seawater has to be evaluated to consider the current status of Fukushima Daiiichi nuclear power plants. However, there is little information about the thermal-hydraulic characteristics of seawater. In order to understand the effects of the seawater on the thermal hydraulic behaviors, a research project was started in Japan Atomic Energy Agency. In this research project, we performed two-different type experiments, one is heat transfer and visualization experiment by using internally heated annulus, the other is a heat transfer experiment by using degraded core simulated test section. The internally heated annulus simulates single fuel rod, simply, and we obtain basic thermal-hydraulics data of seawater in a nuclear reactor. To obtain basic thermalhydraulics data, we performed the PIV and visualization of steam-water interface by using high speed camera. The thermal-hydraulic experiment by using the degraded core simulated test section was performed in order to investigate the effects of seawater to degraded core cooling. In this paper, the outline of this research project, including objective, outline of experimental apparatus and schedule, is reported. In addition, examples of two experimental results are also shown.