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Effect of cooling history on cladding ductility under LOCA conditions

Udagawa, Yutaka  ; Nagase, Fumihisa ; Fuketa, Toyoshi

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.

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Category:Nuclear Science & Technology

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