Neutron Multiplication in the Blanket

One neutron per fusion cannot self-breed after losses; (n,2n) multipliers like beryllium or lead add neutrons before capture to lift net TBR.

Why you need more than one neutron

D–T fusion gives exactly one neutron per reaction, and the plasma burns one tritium per reaction. So even a perfect blanket that bred one tritium per neutron would only break even before any losses. Once ports, structure, and holdup subtract from the budget, one-for-one breeding falls below self-sufficiency.

The fix is neutron multiplication: reactions that turn one incoming neutron into two. The two standard multipliers are beryllium, via ⁹Be(n,2n), and lead, via ²⁰⁸Pb(n,2n). Placed ahead of or within the breeding zone, a multiplier increases the neutron population before capture on lithium-6, which is how a design lifts net TBR above 1.0 with margin.

Multiplier trades

• Beryllium: strong multiplier, but a limited and safety-sensitive material
• Lead: multiplies and also acts as coolant/carrier in some blanket concepts
• Multiplier thickness and placement tune the neutron economy
• Multiplication competes with parasitic absorption for the same neutrons

Role in the reconciliation

Multiplication is the primary tool for recovering breeding lost to real-machine effects, so it is central to closing the local-vs-net TBR gap at the 1.8 target. The achievable gain is a neutronics result under study, not a demonstrated value, and it depends on the final blanket geometry and materials.

This page describes a design-and-simulation study, not a built machine. Construction begins Q2 2027; first-of-a-kind (FOAK) first tritium is targeted near 2030. No net-gain claim is made before FOAK.