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Na-コンクリート反応生成物の熱物性

Thermophysical properties of sodium-concrete reaction products

河口 宗道; 宮原 信哉; 宇埜 正美*

Kawaguchi, Munemichi; Miyahara, Shinya; Uno, Masayoshi*

液体金属ナトリウム(Na)は、高い熱伝導等の特性のため高速炉の冷却材として使われてきた。しかしながら、Na漏えい事故時に鋼製ライナーが破損した場合は、Na-コンクリート反応(SCR)が発生する可能性がある。SCRは、Naとコンクリート成分の化学反応に依存して、コンクリート侵食、水素発生するために、Na漏えい事故時に重要な現象の一つである。本研究では、Naとコンクリート粉末を用いて、SCRに関する基礎的な実験を行った。ここでコンクリート粉末は、日本の原子力発電所の構造コンクリートとして一般的に使われるシリカ系コンクリートを粉末化して使用した。反応過程においては、約100$$^{circ}$$C, 300$$^{circ}$$C, 500$$^{circ}$$CでNaの融解、NaOH-SiO$$_{2}$$の反応、Na-H$$_{2}$$O-SiO$$_{2}$$の反応等の温度変化が観察された。特に、500$$^{circ}$$C近傍での激しい反応においては、Na-コンクリート粉末の混合割合$$gammaapprox0.32$$$$836sim853^circ$$Cの温度ピークが観察され、反応熱は$$0.15sim0.23$$kW/gと推定された。反応生成物の主成分は、X線分析からNa$$_{2}$$SiO$$_{3}$$、融点, 密度, 比熱, 熱伝導率, 粘度等の熱物性値は$$x$$Na$$_{2}$$O-$$(1-x)$$SiO$$_{2}$$ ($$xleq 0.5$$)と同程度であることを確認した。

Liquid sodium (Na) has been used as the coolant of fast reactors for the various merits, such as the high thermal conductivity. On the other hand, it is postulated that a steel liner may fail and lead to a sodium-concrete reaction (SCR) during the Na-leak accident. Because of concrete ablation and release of hydrogen gas due to the chemical reactions between Na and concrete components, the SCR is one of the important phenomena in the Na-leak accident. In the study, fundamental experiments related to the SCR were performed using Na and concrete powder. Here, the used concrete powder is milled siliceous concrete which is usually used as the structural concrete in Japanese nuclear power plants. The obvious temperature changes at 3 temperature regions were observed for the reaction process such as Na-melt, NaOH-SiO$$_{2}$$ and Na-H$$_{2}$$O-SiO$$_{2}$$ reaction, which occurred around 100, 300 and 500$$^{circ}$$C, respectively. Especially, the violent reaction around 500$$^{circ}$$C caused the temperature peak to $$836 sim 853^{circ}$$C, and the reaction heat of $$0.15 sim 0.23$$ kW/g was estimated under the Na-concrete mixing ratio such as $$gammaapprox 0.32$$. The main components of the reaction products was identified as Na$$_{2}$$SiO$$_{3}$$ with X-ray diffraction technique. Moreover, the measured thermophysical properties such as melting point, density, specific heat, thermal conductivity and viscosity were similar to those of $$x$$Na$$_{2}$$O-$$(1-x)$$SiO$$_{2}$$ ($$xleq 0.5$$).

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