The Burner's 5.44% Neutron Fraction

The D-³He burner is low-neutron, not aneutronic: about 5.44% of its fusion energy leaves as neutrons in this design.

The burner (Aegis / MetroVolt) is a D–³He tandem-mirror generator. Its primary reaction, deuterium fusing with helium-3, releases a proton and a helium-4 nucleus and produces no neutron. That charged-particle output is what direct energy conversion captures. But a real D–³He plasma is never pure: deuterium also fuses with itself, and some of that path breeds tritium that then fuses with deuterium at 14 MeV.

In the frozen design point those side reactions carry 5.44% of the total fusion power as neutrons. That number is small, and it is honest. We describe the burner as low-neutron, never as aneutronic. Aneutronic would mean zero neutron output, and no deuterium-bearing fuel can deliver that.

Why the distinction matters

• Neutrons are what activate structural materials; a 5.44% neutron channel is small but real, so activation is reduced, not eliminated.
• Neutrons cannot be steered by magnetic or electrostatic fields, so the fraction that exists must be shielded like any neutron source.
• Calling the machine aneutronic would understate the shielding, activation, and waste engineering the design still requires.

For the same reason, the figure is quoted with its conditions attached — a specific ion temperature and deuterium-to-helium-3 ratio — because it moves if those change. Reporting the operating point alongside the fraction is what lets the number be checked rather than merely asserted.

The practical consequence is favorable: with roughly one part in eighteen of the energy in neutrons, the burner activates far less mass than a pure D–T machine of the same power, and it enables direct conversion. The remaining neutron budget still sets shielding and material choices. This is a design-and-simulation figure for a machine not yet built; it is stated as a target the physics must meet, not a measured result.