Liquid sodium is used as the coolant of the fast reactor because of its high thermal conductivity. It is superior as thermal medium in spite of high chemical reactivity with oxygen. In many cases, the combustion starts after forming the dendrite oxides on its reaction surface in oxygen atmosphere. This fact indicates that sodium doesn’t ignite and burn easily unless dendrite oxides are formed. The understanding of the formation mechanism of the dendrite oxides helps us to obtain the optimum handling of leftover non-burning sodium after the accident. However, experimental knowledge to understand the mechanism of combustion is still insufficient. The purpose of this study is to clarify the oxidization behavior of a liquid sodium droplet precisely [1]. The reason why we choose the droplet form is derived from the fact that the reaction surface can be easily observed. The experiment was carried out in a glove box filled with argon gas. A single sodium droplet was made at the tip of the nozzle and preheated at the experimental temperature. The oxidization started by supplying the gas mixture of nitrogen and oxygen. The oxygen concentration of the gas mixture and the initial sodium temperature were adopted as experimental parameters, 4% ∼ 20% O2 and 200°C ∼ 500°C respectively. When the gas mixture was supplied, the droplet surface was covered with a white oxide layer. Gradually, the dendrite oxides appeared on the lower side of the sodium droplet, and the aerosol was generated in the vapor phase area. Then, the dendrite oxides on the surface sank in the droplet and the surface became smooth again. Finally the sodium droplet was encompassed by orange flame. The dendrite oxides were found to grow bigger at the first reaction period. It is suggested that the dendrite oxides have a porous structure and the liquid sodium in the droplet is drawn up to their tips by capillary force. Then the sodium oxidization occurs at the tips. The sodium droplet covered by dendrite oxides was also found to be heated up and the dendrite oxides sank into the droplet due to the high oxygen solubility and be finally burned due to the attainment of its ignition temperature as the second reaction period. The heat generation of oxidization as the first reaction period contributed to the combustion phenomena.

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