Irradiation growth tests at 553 K and a fast (E > 1.6 × 10⁻¹³ J) neutron flux of about 6.5 × 10¹⁷ n m⁻²s⁻¹ were performed on specimens of Zr-2.5Nb pressure tubes to a neutron fluence of 3.5 × 10²⁵ n m⁻². The tubes were produced by different routes designed to change texture, grain shape, and dislocation density, but X-ray diffraction studies showed only minor changes in texture and dislocation density. However, electron microscopy revealed marked differences in dislocation substructure and second-phase distribution.

All transverse specimens exhibited similar negative growth behavior. However, the longitudinal specimens showed different growth characteristics. Generally a three-stage growth curve was observed showing an initial offset, a low growth rate regime, and a region of apparent steady-state growth. This behavior does not fit with the normal texture, grain shape, and dislocation density dependencies which have previously been reported. An explanation is presented based on partitioning of irradiation-induced point defects, with interstitials attracted to <a>-type dislocations or second-phase boundaries and vacancies migrating to both grain and sub-grain boundaries.