Advocates of Nuclear Fusion often claim that the radioactive  waste in a fusion reactor is much lower than in fission reactors. For example the ITER website says:

No long-lived radioactive waste : Nuclear fusion reactors produce no high activity, long-lived nuclear waste. The activation of components in a fusion reactor is low enough for the materials to be recycled or reused within 100 years. 

The current page gives information on the realities of radioactivity in fusion nuclear reactors.

Neutron Activation

In nuclear fission reactors, the dominant source of radioactivity in from the breakup of the uranium nuclear leading to radioactive isotopes. In fusion the primary fusion reaction leaves no dangerous isotopes but the neutrons emitted from the reaction create radioactive isotope when the hit the surrounding material. This process is called Neutron Activation and is illustrated in the figure below.

Neutron Activation and subsequent decays

The development of the neutron activation is different for every element. For some materials, there are no long lived radioactive isotopes produced while for others there are, with varying half-lives. T

Radioisotopes vs. number of neutrons and number of protons. Colours indicate half-lives: Black-stable, red-years, green/blue-short.

The problems for fusion reactors arise from impurities in metals which lead to a long-lived radioisotope as example:

R&D on Low Activation Materials

To minimize enduring radioactivity in fusion reactors, there has been extensive R&D on materials for three decades. The necessity is to avoid elements which are activated to isotopes with long lifetimes. This has involved finding alloys which avoid certain elements as for instance nickel because of their place in the isotope picture above. It has been possible to find steel alloys which compared to conventional steel which, after irradiation in a fusion reactor, would need 200 000 years to decay to "low-level waste" criteria, would only take 100 years.

Levels of Radioactivity

The levels of radioactivity in fusion reactors are being estimated in reactor design studies. The task is complicated because of the range of materials needed to satisfy all requirements . The figure below estimates the radioactivity levels in part of the vacuum vessel at the end of DEMO operations from : Activation, ... European DEMO concept . This longest lasting radioactivity is due to the Nickel-63 isotope (100 years half-life) and carbon-14 (5000 years half-life) which in this example is at a concentration but in some materials is significant.

The figure below from:  Waste assessment of European DEMO fusion reactor designs show the timescales at which the radioactivity has decay so the components can be considered Low Level Waste (LLW). It can be seen that the blanket modules, the divertor and the inner part of the vacuum vessel are radioactive for more than 1000 years and so do not meet the claim for no-long lived radioactive waste quoted above from ITER.

There is significant uncertainty in what will be the real radioactivity levels because everything depends on the materials used and especially on the impurities in the finally delivered materials. 

Quantity of Radioactive Waste

Fusion reactor have a much great amount of material which is subjected to a flux of neutron than comparable power fission reactors. Correspondingly, the volume of radioactive waste is much greater although with lower levels and shorter lifetimes. The plot below makes illustrative comparisons.

Fusion power plants will have higher volumes of radioactive waste than fission power plants but the activity will be lower with little "high-level waste". An analysis of radiotoxicity of fusion reactors compared to fission reactors, shows fusion reactors will have higher levels of short-lived radionuclides than fission reactors but the situation reverse over time because of the decay. For instance, after 100 years the radiotoxicity of fusion reactors becomes 100 times lower than fission reactors and after 500 years the radiotoxicity of fusion reactors is close to natural radiotoxicity, such as in the fly ash near coal plants.