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

Udagawa, Yutaka  ; Nagase, Fumihisa ; Fuketa, Toyoshi

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 1717-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 phase in the radial cross section of the cladding, and reduced cladding ductility. Slow-cooling rate decrease increased unit size and hardness of precipitated phase, while phase area fraction and cladding ductility were not significantly changed. 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.