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Study on transient void behavior during reactivity initiated accidents under low pressure condition; Development and application of measurement technique for void fraction in bundle geometry

低圧条件における反応度事故時過渡ボイド挙動に関する研究; バンドル体系内ボイド率計測手法の開発と適用

佐藤 聡; 丸山 結; 浅香 英明; 中村 秀夫

Satou, Akira; Maruyama, Yu; Asaka, Hideaki; Nakamura, Hideo

原子力機構では、燃料の一層の高燃焼度化に備え、反応度事故時におけるボイド反応度フィードバックを考慮した、最大燃料エンタルピのより現実的な評価に資する知見の取得を目的に、炉外過渡ボイド挙動試験を実施している。本報では、バンドル体系の長尺試験体を用いた、低圧の低温零出力条件を模擬する試験(低圧・長尺試験)を実施するにあたり、バンドル体系内のボイド率の過渡変化を計測する手法を開発し、電場解析,模型試験及び定常沸騰試験により、その適用性を確認した。さらに低圧・長尺試験体を用いた過渡ボイド試験に本ボイド率計を適用することにより、バンドル内における過渡ボイド挙動の、ヒーターピン出力及びサブクール度への依存性に関する知見を取得した。

Series of out-of-pile experiments to obtain the knowledge on the transient void behavior during reactivity initiated accidents are in progress at JAEA. In the present series of experiments, the transient void behavior in a test section of 2$$times$$2 bundle geometry under atmospheric pressure condition was measured using an impedance technique. The measuring areas and the arrangement of electrodes for the impedance technique were defined on the basis of numerical analyses and scaled model experiments. The comparison was made between the impedance and differential pressure techniques for steady boiling experiments to estimate the accuracy of the impedance technique. The impedance technique showed a good agreement with the void fraction estimated from the differential pressure. The transient void behavior in the bundle geometry was measured using the impedance technique. It was clarified that the transient void behavior depends on both the subcooling of inlet water and the heat generation rate of simulated fuel rod. Local void fraction was influenced by the ratio of flow area to heat transfer area of the simulated fuel rod.

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