Effect of cooling history on cladding ductility under LOCA conditions

LOCA時被覆管延性低下に及ぼす冷却時温度履歴の影響

宇田川 豊  ; 永瀬 文久 ; 更田 豊志

Udagawa, Yutaka; Nagase, Fumihisa; Fuketa, Toyoshi

軽水炉の冷却水喪失事故において、燃料被覆管は崩壊熱により酸化され、徐冷(ゆっくりと冷却)された後、再冠水により急冷される。徐冷時の冷却速度と急冷開始温度が急冷後の被覆管の延性に及ぼす影響を調べた。試験では、未照射ジルカロイ-4被覆管試料が水蒸気中で1373及び1473Kの高温で酸化され、次いで27K/sの冷却速度で徐冷され、最後に1073K1373Kに達した時点で急冷された。急冷後の延性をリング圧縮試験により、ミクロ組織を金相試験と硬さ試験,酸素分析により調べた。冷却時温度履歴により、prior-相中に析出した高酸素濃度の相組織が変化した。析出した相領域の、被覆管断面における面積率は、低い温度で急冷された試料で大きくなり、被覆管の延性は減少した。低延性の相領域は亀裂伝播の経路となりうるので、相領域の面積率増大が破断を容易にし、延性を低下させる可能性がある。一方、徐冷過程の冷却速度を変化させても相領域の面積率はほぼ一定であり、冷却速度が被覆管急冷後の延性に及ぼす影響は無視しうる程度であった。

In a Loss-of-Coolant Accident of LWRs, heated and oxidized fuel cladding is slowly cooled and then quenched by re-flooding water. Effects of cooling rate during the slow-cooling process and quench temperature were investigated on post-quench ductility of the oxidized cladding. In the tests, unirradiated Zircaloy-4 cladding specimens were oxidized in steam at 1373 or 1473K, cooled at a rate from 2 to 7 K/s, and finally quenched from temperatures ranging 1073 to 1373 K. Post-quench ductility was evaluated by ring-compression test, and microscopic properties were examined by metallurgical examination, hardness test, and oxygen analysis. Morphology of oxygen-rich alpha-phase in the metallic prior-beta phase layer changed depending on the cooling history. Area fraction of alpha-phase region in the cross section was obviously increased and post-quench ductility was reduced when the quench is initiated at lower temperatures. Since alpha-phase region with lower ductility can be preferential path for crack propagation, increase in area fraction of alpha-phase region possibly decreases resistance for fracture, which results in ductility reduction. On the other hand, the area fraction was nearly constant irrespective of the rate of the slow cooling, and consequently, effect of the cooling rate on the post-quench ductility was negligible.