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冷却材喪失事故時の被覆管延性低下に及ぼす冷却時温度履歴の影響

Effects of temperature history during cooling process on cladding ductility reduction under lost of coolant accident conditions

宇田川 豊; 永瀬 文久; 更田 豊志

Udagawa, Yutaka; Nagase, Fumihisa; Fuketa, Toyoshi

急冷開始温度及び急冷前の冷却速度がLOCA時の被覆管延性低下に及ぼす影響を調べることを目的とし、未照射PWR用17$$times$$17型ジルカロイ-4被覆管から切り出した試料を水蒸気中、1373及び1473Kで酸化し、ゆっくりと冷却(徐冷)してから急冷した。試験条件のうち、徐冷の速度を2$$sim$$7K/s、急冷開始温度を1073$$sim$$1373Kの範囲で変化させて複数の試験を行い、冷却条件の異なる試料を得た。酸化,急冷した試料に対しリング圧縮試験,ミクロ組織観察,ビッカース硬さ試験を実施した。急冷開始温度低下に伴い、金属層中に析出する$$alpha$$相の面積割合が大幅に増加し、被覆管の延性が明確に低下した。徐冷速度の減少に伴い、析出した$$alpha$$相の単位大きさ及び硬さの増大が生じたが、面積割合及び被覆管の延性はほとんど変化しなかった。析出$$alpha$$相は周りの金属層より硬く、また酸素濃度が高いことから、その延性は非常に低いと考えられる。したがって、析出$$alpha$$相の面積割合増大が、急冷開始温度低下に伴う延性低下促進の近因である。

In order to investigate effects of quenching temperature and cooling rate before quench on cladding ductility reduction under LOCA conditions, samples cut from non-irradiated 17$$times$$17-type Zircaloy-4 cladding tubes for PWRs were oxidized in steam at 1373 and 1473 K, cooled at 2 to 7 K/s, and quenched at 1073 to 1373 K. The quenched samples were subjected to ring compression test, microstructure observation, and Vickers hardness test. Quenching temperature decrease obviously increased area fraction of $$alpha$$ phase in the radial cross section of the cladding, and reduced cladding ductility. Slow-cooling rate decrease increased unit size and hardness of precipitated $$alpha$$ phase, while $$alpha$$ phase area fraction and cladding ductility were not significantly changed. $$alpha$$ phase is harder than the surrounding region in the metallic layer and has higher oxygen content, indicating its low ductility. Consequently, increase in the area fraction in the cladding is a main cause of the reduction in cladding ductility with decrease in the quenching temperature.

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