[en] The purpose of this basic experiments to clarify the cause of damage of the liner material, which recognized by the Sodium Leak and Combustion Test II. In this experiment, the liquid sodium continuously dropped into the high-temperature NaOH molten pool, to investigate sodium combustion phenomenon in the NaOH pool and damage of the liner material (carbon steel, JIS, G3106, SM400B). The drop temperature of sodium is 496degC, and the amount of dropped sodium is about 1200g. The temperature of NaOH molten pool is 660-690degC in the initial stage, and the amount of NaOH is about 2500g. The average drop (leak) rate is 1.8g/sec, and the height of above the NaOH molten pool is about 700 mm. The following results were obtained. The fallen sodium continued to burn during floating on the NaOH molten pool. In this case, the NaOH molten pool was not totally covered with the combustion products, and the molten pool always had the atmosphere interface. The combustion products were mixed and melted in the NaOH molten pool. The maximum reduction of thickness was occurred in the level vicinity of NaOH molten pool. Plate test specimens of 3 mm thickness were separated in the level vicinity during periods of combustion. And, in the NaOH container, the metal loss of largest about 2.5 mm was recognized at the level vicinity. It is considered that the remarkable metal loss in the level vicinity indicates the involvement of molten salt-type corrosion mechanism with the atmosphere interface. The above mentioned results were obtained from the basic experiment. These results gave the important information to evaluate the damage mechanism of the floor liner material. (author)