The 5.44% Neutron Fraction

Only 5.44% of the burner's fusion power leaves as neutrons — the number that lets the machine skip heavy shielding and the steam cycle.

A small but real number

The dominant D-3He reaction is aneutronic, but the deuterium in the fuel also undergoes side D-D reactions, one branch of which makes neutrons, plus tritium that can burn to a 14 MeV neutron. Summed over the design-point plasma, these put 5.44% of total fusion power into neutrons. Compare a D-T machine, where roughly 80% of the power is neutrons.

What a low fraction buys

• Light shielding instead of a thick blanket
• Low material activation, so a thin radioactive inventory
• Most power stays charged for direct conversion
• Reduced neutron damage to the first wall and magnets, aiding component life

Still handled honestly

5.44% is small, not negligible. The neutrons still deposit heat and cause some activation, so the burner carries a modest shield and recovers the neutron heat thermally through waste-heat management. The tritium bred by D-D side reactions is captured and managed, not released — see tritium management.

This one figure is what separates a burner sited beside a data center from a conventional fusion plant that needs a thick blanket, a large exclusion zone, and a thirsty cooling system. The whole co-location case rests on the fuel choice that produces it.

The figure also shapes licensing: a plant whose radiological source term is dominated by a thin activated layer and a small tritium inventory is a fundamentally different regulatory object than a D-T machine, and the burner's safety case is built on that difference.