= Solution
The low-temperature zirconium matrix has a <hexagonal close packing> structure. Its crystallographic $a$ and $c$ directions therefore have different <elasticity>, <thermal expansion> and available <slip systems>. Producing tubing also develops a <crystallographic texture>, so averaging over grains does not necessarily remove these directional effects.
Different expansion along the crystal axes produces intergranular thermal <stresses> during <temperature> changes. Under load, easy <dislocation glide> modes may accommodate some strains much more readily than others; additional slip or <crystal twinning> is needed for general compatibility. Consequently strength and <ductility> can depend on the axial, circumferential and radial orientation of the tube.
<Irradiation> creates <vacancies> and <self-interstitials> that cluster into <dislocation loops> with different crystallographic orientations. Their anisotropic accumulation can produce <irradiation growth> even without external <stress>. Different directions can elongate or contract while the net volume change remains small. Texture turns these grain-scale strains into macroscopic length and diameter changes, bowing and <stresses> between constrained components. This mechanism is distinct from the added cavity volume of <void swelling>.
The crystallographic character of <irradiation> loops matters: atomistic calculations by <Rory Hulse> and <Christopher P. Race> examine several possible loop types and show that loop energy and shape depend on orientation and size. A single generic spherical-defect description therefore misses important aspects of zirconium's dimensional response.<An atomistic modelling study of the properties of dislocation loops in zirconium by Rory Hulse and Christopher P. Race>[Abstract and loop-orientation results; the study is atomistic modelling, not a direct measurement of every loop population]{ref}
Hydrogen also produces direction-sensitive damage. <Zirconium hydrides> are brittle precipitates whose preferred orientation depends on crystal texture and <stress> history. Radially oriented hydrides can offer crack paths across a thin tube wall under circumferential tension. Controlling texture, hydrogen pickup and hydride orientation helps manage that risk; changing the hexagonal structure's orientation cannot by itself eliminate <corrosion> or <irradiation> damage.
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