neutron activation is the process by which neutron capture makes reactor materials radioactive: a stable nucleus absorbs a neutron, becomes an unstable isotope, and decays over a timescale set entirely by which element it was.
Activation is the near-neighbor of 📝neutron damage, and the two are routinely conflated: the same bombardment produces both, but damage degrades a material's mechanical performance while activation changes its nuclear identity, leaving components radioactive after the machine shuts down. Damage limits how long a part survives; activation determines who can approach it and what its disposal costs.
The distinction matters most in what 📝fusion does not produce. A fusion plant generates no fission products and no actinides — the long-lived, chemically mobile species that dominate fission waste. Its only significant radioactive legacy is the activated structure around the plasma, and that legacy is a design variable rather than a fact of physics: the 14 MeV neutrons of the 📝D-T reaction activate whatever they strike, but how long the radioactivity lasts depends on the elements in the material. Nickel, niobium, molybdenum, and cobalt transmute into isotopes with half-lives of centuries or more; iron, chromium, vanadium, and tungsten decay to safe handling levels within decades. Choosing low-activation materials is therefore the main lever by which fusion keeps its waste short-lived — and the reason activated in-vessel components are maintained by remote handling rather than by hand.