Effect of microstructural evolution on in-reactor creep of Zr–2.5Nb tubes

Dislocation density, decomposition of the β-Zr phase and diametral creep were examined as a function of the location of the Zr–2.5Nb tube irradiated in the Wolsong Unit 1 for 9.3 effective full power years (EFPYs). The maximum a-dislocation density occurred at the inlet part of the irradiated Zr–2.5Nb tube exposed to the lowest temperature while the outlet part of the tube exposed to the higher temperature had the higher extent of decomposition of the β-Zr phase and the maximum diametral creep. Thus, it is concluded that in-reactor creep of the Zr–2.5Nb tube is not related to the dislocation density but governed by the Nb concentration of the α-Zr grains caused by thermal decomposition of the β-Zr phase. Supplementary creep tests on the Zr–2.5Nb sheets with different Nb contents in the β-Zr phase provide supportive evidence to this conclusion. The acceleration of the in-reactor creep of the Zr–2.5Nb tubes is suggested after a long-term operation.