Thermal creep behavior of hydrogen-enriched Zr1%Nb alloy fuel cladding tube in phase transit regions.

Among various degradation processes, thermal creep is considered a life-limiting factor for zirconium nuclear fuel cladding in light water reactors. In this work, the creep behavior of pre-oxidized and pre-hydrided in steam thin-walled Zr1wt.%Nb alloy fuel cladding tube was investigated in α-Zr → (α+β)-Zr → β-Zr phase transit regions to further develop knowledge on acting creep deformation mechanisms and the role of dissolved hydrogen. Short-term constant-stress creep tests in tension were conducted on pre-hydrided tubular segments of the cladding tube over a temperature range of 550-900 °C and at applied stress of 5 to 60 MPa. The hydrogen contents in creep specimens before creep tests were 150 and 600 wppm, respectively. Creep tests were followed by microstructural analysis of the specimens using scanning and transmission electron microscopy. The activation analysis of the creep data indicated that the dominant creep deformation mechanism during the phase transition may be dislocation glide, which is not altered by oxygen or hydrogen. However, the oxygen and hydrogen influence its kinetics. The dominant creep-hardening effect in the α-Zr → β-Zr transition provides oxidation hardening. Dissolved hydrogen does not directly contribute to creep hardening.