neutron damage is the structural degradation a material suffers under sustained neutron bombardment, as collisions knock atoms out of their crystal lattice — a central materials-qualification challenge for fusion power plants.
Neutron damage is distinct from neutron activation, its constant companion: activation changes what a material is, transmuting stable nuclei into radioactive ones, while damage changes how it performs — hardening, embrittlement, swelling, and irradiation creep that erode the strength and toughness engineers designed in. The same bombardment produces both, but they are managed by different means: activation by choosing the right elements, damage by qualifying materials to survive a known dose, counted in displacements per atom.
The 14 MeV neutrons of the 📝D-T reaction are far more energetic than fission neutrons, and each collision sets off a cascade of thousands of secondary displacements. They also drive transmutation reactions that generate helium and hydrogen inside the metal itself; the gas collects into bubbles at grain boundaries and embrittles the material in ways displacement alone does not. Because no operating facility yet reproduces this combination of high 📝neutron flux, 14 MeV spectrum, and internal gas production, qualifying materials for fusion conditions remains one of the field's defining open problems — the reason dedicated irradiation sources and conservative, replaceable component designs both feature in fusion power plant planning.