 Solution

Energetic neutron collisions create displacement cascades and Frenkel pairs: displaced atoms become self-interstitials, leaving vacancies. During migration, some opposite defects recombine, while others cluster or reach sinks. Interstitials can condense into extra atomic planes bounded by dislocation loops; vacancy clustering can instead give missing-plane loops. Their subsequent growth is driven by defect absorption and dislocation climb.

Randomly oriented loops do not select a macroscopic shear direction in an initially isotropic polycrystal. That does not prove that every external dimension remains unchanged: interstitial and vacancy loops have opposite relaxation-volume contributions, and an imbalance can change the volume. Macroscopic void swelling requires the additional vacancy-cavity mechanism; it must not be identified automatically with the mere presence of loops. Texture, stress-biased absorption and unequal loop populations can also give anisotropic dimensional change.