Effect of microstructural evolution on in-reactor creep of Zr–2.5Nb tubes YoungSuk Kim * , KyungSoo Im, YongMoo Cheong, SangBok Ahn Korea Atomic Energy Research Institute, Zirconium Group, P.O. Box 105, Yusong, Daejon 305-353, Republic of Korea Received 22 July 2004; accepted 4 June 2005 Abstract Dislocation density, decomposition of the b-Zr phase and diametral creep were examined as a function of the loca- tion 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 b-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 a-Zr grains caused by thermal decomposition of the b-Zr phase. Supplementary creep tests on the Zr–2.5Nb sheets with different Nb contents in the b-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. Ó 2005 Elsevier B.V. All rights reserved. PACS: 61.80.Hg 1. Introduction When Zr–2.5Nb pressure tubes are operated in reac- tors, their microstructures evolve with neutron irradia- tion. Thus, it is very important to understand how their microstructures evolve with operational conditions such as fast neutron fluences and temperatures and what effects they have on the long-term degradation of pres- sure tubes. By investigating the effect of the microstruc- tural evolution of Zr–2.5Nb tubes on corrosion, Urbanic reported that the b-Nb particles precipitated in the a-Zr grains is a governing factor to the lower cor- rosion of Zr–2.5Nb tubes under neutron irradiation [1]. However, no study has been undertaken so far on the ef- fect of microstructural evolution on the in-reactor creep of Zr–2.5Nb tubes. Recently, Griffiths just reported that a big scatter in the creep rates of Zr–2.5Nb pressure tubes is largely due to variations in grain size, texture and oxygen content [2]. Though recognizing that the pressure tubes have their microstructures evolving dur- ing operation in reactors [3], he did not consider its effect on their creep and resulting scatter in the creep rates [2]. There are two factors to cause a microstructural change in the Zr–2.5Nb tubes: one is the fast neutron fluence (E > 1 MeV) and the other is temperature. The highest 0022-3115/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2005.06.004 * Corresponding author. Tel.: +82 42 868 2359; fax: +82 42 868 8346. E-mail address: yskim1@kaeri.re.kr (Y.S. Kim). Journal of Nuclear Materials 346 (2005) 120–130 www.elsevier.com/locate/jnucmat