Every "hit" knocks an atom off its lattice site, leaving behind a vacancy and a nearby interstitial — a mobile point-defect pair (the orange dots). Each defect then random-walks through the crystal. If it wanders near a grain boundary (the fine mesh lines), the boundary's disordered, open structure absorbs and annihilates it — it simply vanishes, harmlessly. If it never finds a boundary, it eventually gets trapped and starts nucleating a void; once a void exists, further defects are captured by it too, so damage self-accelerates — that's swelling.
grain-boundary area / volume ∝ 1 / grain size
sink strength ∝ boundary area/volume
P(defect reaches a sink) ↑ as grain size ↓
- Irradiation rate — how many knock-on events fire per second on both samples simultaneously, i.e. the neutron/ion flux.
- Nanocrystalline grain size — cell-spacing of the grain-boundary mesh on the right sample; shrink it and boundary density (sink area per volume) rises, so more defects are absorbed before they can cluster.
- Coarse-grained swelling / Nanocrystalline swelling — fraction of lattice sites locked into growing voids on each sample, for the identical dose. The gap between the two numbers is exactly the payoff of nanostructuring for radiation tolerance.
- Defect-sink efficiency — share of all spawned defects that were absorbed by a grain boundary rather than ending up in a void.
Real-world relevance: this is why nanocrystalline alloys and oxide-dispersion-strengthened (ODS) steels are studied for next-generation fission and fusion reactor components — their huge internal grain-boundary area soaks up radiation damage that would otherwise swell and embrittle a coarse-grained structural material over decades of service.