Transmutation and the Nuclides It Creates

Neutron capture and knock-out reactions transmute one element into another; the resulting nuclide chart is the fusion waste inventory.

Transmutation is the change of one element into another by nuclear reaction. In a fusion machine it happens continuously as neutrons interact with structural atoms. The inventory of new nuclides created this way — not any spent fuel — is the radioactive waste the plant must eventually manage.

The main routes

• Capture (n,γ): A(Z) + n → A+1(Z), often radioactive, e.g. ⁵⁸Fe → ⁵⁹Fe.
• Proton knock-out (n,p): changes the element down one, e.g. ⁵⁴Fe(n,p)⁵⁴Mn, and makes hydrogen.
• Alpha knock-out (n,α): changes the element down two and makes helium, e.g. the intended ⁶Li(n,α)T.
• (n,2n): at 14 MeV, ejects a second neutron and can open long-lived channels.

Two secondary effects of transmutation matter for engineering beyond radioactivity: (n,p) and (n,α) reactions deposit hydrogen and helium into the metal lattice, which causes swelling and embrittlement over a component's life. So transmutation both creates the waste nuclides and helps set when a component must be retired.

Because the products are set by the starting elements and the spectrum, activation can be computed in advance with the same nuclear-data libraries used across the field. That predictability is what makes the waste class a design output — chosen by material and geometry decisions — rather than something discovered only after a machine has run.

Because the nuclides that result are fully determined by the starting elements and the neutron spectrum, the waste inventory is predictable and controllable at the design stage. Choose the elements, know the spectrum, and the nuclide chart — and therefore the waste class — follows. All of this is design-and-simulation work for machines not yet operating.