[en] To understand physical and thermal characteristics of liquid sodium, to be experienced in sodium handling and to achieve a capability of preliminary step on LMFBR safety, a study for liquid sodium used as a coolant of LMFBR was carried out. This study includes the state of the arts of critical heat flux, FBR steam generators and measurement techniques. Experiments of sodium-steam reaction and of oxygen measurement in sodium using vanadium wire were also carried out. According to the result obtained by sodium-steam experimens, no significant aggresive explosion phenomena were observed as compared with that of sodium-water reactions. It was also observed that the rate of increase of reaction temperature decreases with increasing initial temperature of liquid sodium and that the temperature of the target depends upon the flowrate of injected steam. By the experiment of oxygen measurement, vanadium wire became very brittle and was decreased in weight of large amount after exposure to liquid sodium. Oxygen concentration in vanadium increased from 566ppm to 1.6118 wt% under given experimental conditions. (Author)