A burning fusion plasma subjects the confining plasma vessel to strong loading from plasma particles, neutrons and electromagnetic radiation. Charged and neutral plasma particles affect the plasma vessel mainly close to the surface. This can lead to physical or chemical sputtering, which, however, can be appreciably restricted by divertors, and to thermal loading. The stresses thereby induced must be taken into account when specifying the mechanical strength in designing the device. Most important for material performance is the neutron load. The fusion neutrons have a considerable range, penetrate the first wall and blanket and transfer their energy to them. This involves displacement of lattice atoms and nuclear transformations causing swelling, creepage and embrittlement of the material.
The objective of materials development is not, however, just to achieve the highest possible resistance to such loads; the materials should also be activated as little as possible by the fusion neutrons or this activation should decay as fast as possible.