The Neutron Multiplier

One fusion neutron cannot breed more than one tritium; a neutron multiplier makes extra neutrons so the breeder can run tritium-positive.

The arithmetic problem

Each D-T fusion produces one neutron and burns one tritium. Breeding one tritium per neutron would, after unavoidable losses and leakage, leave the machine tritium-negative. To breed more tritium than it burns, the blanket needs more neutrons than fusion provides. That is the job of the neutron multiplier.

How multiplication works

Multiplier materials — beryllium-based or lead-based — undergo reactions that release two neutrons for one absorbed at fusion energies. This raises the neutron population entering the lithium and lets the tritium breeding ratio exceed one. Multiplier choice, placement, and quantity are central to whether a target TBR of 1.5 or 1.8 is reachable.

• (n,2n) reactions turn one fast neutron into two.
• Beryllium- and lead-based multipliers are the main candidates.
• Placement between first wall and breeder maximizes effect.
• Multiplier inventory and activation are materials considerations.

Why it is not free

Multipliers add material, cost radial space, and bring their own activation and supply questions (beryllium supply and handling, lead compatibility). They are essential to a TBR above one but are part of the coupled blanket trade, not a bolt-on. The realistic multiplication achievable at full coverage feeds directly into the open TBR reconciliation.

Activation and handling

Multiplier materials bring their own activation and handling considerations: beryllium carries supply and toxicity constraints, while lead-based multipliers raise compatibility and mass concerns. These downstream consequences, together with radial-space cost, mean the multiplier is chosen against the whole blanket and maintenance picture, not solely its neutron yield.

This page documents a design and simulation study, not a built machine. Construction begins Q2 2027; first-of-a-kind first tritium is targeted near 2030. Figures are computed, reproducible targets, not measurements.