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How can neutron absorption make materials around a fusion reactor radioactive?

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laser-activated fusion — nuclear fusion Interior of the U.S. Department of Energy's National Ignition Facility (NIF), located at Lawrence Livermore National Laboratory, Livermore, California. The NIF target chamber uses a high-energy laser to heat fusion fuel to temperatures sufficient for thermonuclear ignition. The facility is used for basic science, fusion energy research, and nuclear weapons testing.

In a deuterium-tritium (D-T) fusion reactor, the fusion process produces high-energy neutrons as a primary by-product. Because these neutrons lack an electric charge, they are unaffected by the magnetic fields used to confine the plasma and can escape to strike the surrounding reactor materials.12

When these neutrons are captured by the nuclei of the surrounding structural materials, they can trigger nuclear reactions that transform stable atoms into new, unstable isotopes. This process of neutron absorption often results in the creation of radioactive by-products. While these materials become radioactive, engineers mitigate this issue by utilizing "low-activation" materials—such as specific types of steel or ceramic composites like silicon carbide—which ensure that any resulting radioactive waste has significantly shorter half-lives and lower toxicity compared to the waste generated by traditional fission reactors.31