Hydrogen behaviors in steam, which seem to be main reason of explosion in the nuclear power plant, were investigated by simulating hydrogen to helium. Hydrogen generates by both radiolysis and water-Zr reaction. Hydrogen behaviors in vessel or pipe have not been well clarified in spite of its importance. To avoid similar explosion in the next generation plants, steam-helium fundamental mixing behaviors were clarified experimentally and hydrogen concentration mechanism, which increased as high as 4%, was studied. From experimental results, steam and helium was well mixed with perfect insulation, while these gases seemed to be separated with local heat loss.