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Embrittlement and fracture behavior of pre-hydrided cladding under LOCA conditions

冷却材喪失事故条件下における水素吸収被覆管の脆化と破断挙動

永瀬 文久; 更田 豊志

Nagase, Fumihisa; Fuketa, Toyoshi

原研では高燃焼度燃料のLOCA時挙動を調べる体系的な研究計画を進めている。同計画の一環として再冠水時に燃料棒が急冷される際の破断限界を明らかにするため、燃焼度39$$sim$$44GWd/tの照射済PWR燃料から採取したジルカロイ-4被覆管を対象に急冷時耐破断特性試験を実施した。破断限界は被覆管が吸収している初期水素量の増加によって低下するものの、照射済燃料被覆管と水素を吸収させた未照射被覆管との間で明らかな違いは見られなかった。また、あらかじめ酸化・水素吸収させ、急冷を経た被覆管に対するリング引張及び圧縮試験を通じて急冷による延性の低下について調べ、欧米で規制に用いられている延性ゼロ基準は、急冷時破断特性試験に比べてより保守的な結果を与えることなどを示した。

A systematic research program on high burnup fuel behavior under LOCA conditions is being conducted at JAERI. As a part of the program, integral thermal shock tests simulating the whole LOCA sequence were conducted with Zircaloy-4 fuel claddings, irradiated to 39 and 44 GWd/t at a PWR, to investigate behavior and condition of cladding fracture during quenching for safety evaluation. Differences were not clearly observed between irradiated and unirradiated claddings at similar hydrogen concentrations in terms of threshold of fracture during quenching, though the threshold is reduced as initial hydrogen concentration increases. Ductility of pre-hydrided, oxidized and quenched claddings was also evaluated by using ring-tensile and ring-compression tests. Embrittlement criteria (zero-ductility limits) from both the tests were lower than the fracture conditions in the integral thermal shock tests. This indicates that loading conditions should be well simulated to evaluate cladding performance under LOCA conditions.

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