An international consortium aims to harness nuclear fusion for electric power generation.
Materials Under Fire
Materials close to the plasma in a fusion-power reactor are bombarded by high-energy neutrons yet must produce as little radioactive waste as possible. Thus, low-activation materials those with a limited propensity to become radioactive under neutron bombardment are a must if fusion is to become a viable energy source. ITER will typically produce damage of 3 dpa (displacements per atom) in the austenitic stainless steel of the first containment wall. With judicious use of low Nb and Co grades of steel, most radioactive waste in ITER (except part of the vessel and its internal components) will be cleared for unrestricted reuse a century after decommissioning.
For commercial power reactors, damage to the first material walls if made of stainless steel would be approximately 300 to 500 dpa over a 30-year life. Even if the walls could be changed every few years, this amount of damage is beyond the capability of austenitic steels, which significantly swell above damage levels of 30 dpa.
Materials that last longer or experience less damage will thus be needed. Some promising candidates, low-activation ferritic steels and SiC composites, withstand more than 150 dpa without swelling.
Despite the relatively low damage rate, ITER will be the first facility where materials face a true fusion-neutron spectrum. How components stand up is of great interest. For instance, diagnostic systems must maintain optical, electrical, and structural properties under high radiation doses. Plasma-facing materials must ensure plasma purity and efficiently remove heat. Joints to underlying heat sinks and structures, and the cooling system, will also be scrutinized.
The magnet support structure, on account of its large size, must be assembled using welds. This introduces weak points liable to fatigue. And how the superconductors behave in operation is a key consideration.
By far the most widely used structural material in ITER is austenitic stainless steel, which has largely been qualified for nuclear use through fission and fast-breeder development programs. Beryllium, tungsten, and carbon-fiber composite are used in the first wall facing the plasma. These materials join to copper-alloy heat sinks and, in turn, to stainless-steel supports.
Operation will include development and testing of lithium-based, tritium-breeding blankets essential for tritium self-sufficiency in future fusion-power reactors. These blankets may be connected to a turbogenerator and generate electricity from fusion for the first time.