Radiation Damage is the degradation of the mechanical properties of solid materials caused by radiation. In nuclear reactors this damage is mainly caused by neutrons and is a more serious problem for fusion reactors than fission reactors because of the higher energy of the neutrons.
For metals, this damage generates embrittlement caused by atoms being knocked out of their sites in the lattice structure.
For fusion reactors much research is in progress to find metals resistant to radiation but fusion power plants include plans to regularly replace components when their properties become too degraded.
Basics details on the physics can be found on the page: Essentials of Nuclear Physics
Details on the effects of neutron damage of reactor components can be found in the page: Materials and Radiation Damage