Oxidation behavior of liquid sodium droplet before combustion; Dependency of initial temperature and oxygen fraction

ナトリウム液滴の燃焼前における酸化反応挙動; 初期温度と酸素濃度依存性

西村 正弘 ; 上出 英樹 ; 杉山 憲一郎*; 大竹 志朗*

Nishimura, Masahiro; Kamide, Hideki; Sugiyama, Kenichiro*; Otake, Shiro*

高速増殖炉の冷却材に使用されている液体ナトリウムは冷却材として優れた熱的特性を有する反面、酸素や水との反応が活性である。事故時に想定されるこれらの反応に対して必要な安全対策がプラントでは施されているが、もんじゅのナトリウム漏えい事故などを背景に、特定の事故パターンによらずに反応現象の理解を深め、機構論的に解析することが社会的受容性の観点から求められている。また、反応現象の理解を深めることは、安全対策という観点以外に、事故,トラブルが起こってしまった場合に残存する未反応ナトリウムの取り扱いにおける判断根拠という観点からも、重要な研究テーマである。従来、液滴燃焼の酸化現象において柱状酸化物が生成した後に着火に至ることが観察されているが、十分な現象の理解はされていない。本報では、液滴状のナトリウムが燃焼する際の反応表面を反応雰囲気の酸素濃度や初期の予熱温度をパラメータとして表面の柱状生成物の生成過程に着目して観察した実験結果を報告する。

Liquid sodium is used as the coolant of the fast reactor (FR), because of its high thermal conductivity and wide temperature range of liquid phase. It is superior as thermal medium, however the chemical reactivity with water and oxygen is very high. Hence FR plants have been taking safety measures for these reactions. After the "Monju" sodium leak accident, it is desired that more physical and chemical understanding of reaction phenomena and more mechanistic analysis for the sodium fire from the view point of public acceptance. The purpose of this study is to understand oxidation behavior of a liquid sodium droplet precisely, which is a fundamental reaction of spray fire and is easy to observe the reaction interface. This study is also useful for the establishment of safety criterion to handle the remained non-burning sodium after the accident. The oxidation of a liquid sodium droplet was visualized by using a simple experimental setup and a high speed video camera. A sodium single droplet of ca. 50 mg was made at the tip of a nozzle in a combustion chamber. The oxidation was started by supply of oxygen and nitrogen mixture gas. The initial temperature of sodium droplet and the oxygen fraction in the atmosphere were selected as experimental parameters. It was shown that columnar oxides grew longer as initial temperature of sodium droplet was lower and oxygen fraction was lower. In addition, it was observed that sodium combustion with an orange light emission started from the tip of columnar oxides grown out from the droplet surface. These observations suggest the existence of mechanism that liquid sodium is drawn up from droplet to reaction interface by the capillary force caused in the porous oxides which are formed on the droplet surface.