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.