= Solution
The characteristic growth phenomena require an asymmetric material response and a way to retain irreversible change. \b[Low crystal symmetry and directional defect or deformation behaviour make <uranium> and <zirconium> important examples], whereas an ideal cubic material has isotropic linear <thermal expansion> and no symmetry-selected traceless single-crystal growth tensor in an otherwise isotropic environment.
<alpha uranium> is orthorhombic and has strongly anisotropic thermal response. In a textured <polycrystalline material>, incompatible <crystal grain> <strains> can be accommodated asymmetrically over a thermal cycle and leave macroscopic distortion. The same metal can also show <irradiation growth> through anisotropic irradiation-defect evolution. <zirconium> and its hexagonal close-packed alloys are major examples of <irradiation growth>; their basal and c-axis directions need not acquire the same <strain>.
Cubic <austenitic steel> does not exhibit the same intrinsic orientation-driven response merely because it is irradiated. It can nevertheless undergo <void swelling>, <irradiation creep>, thermal-stress <plasticity (physics)> or distortion under gradients. Thus \b[absence of this growth mechanism does not imply dimensional stability under all reactor conditions]. Conversely, a noncubic crystal is not guaranteed to grow during every thermal cycle: constraints, inelastic accommodation and the <microstructure> still matter.
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