The 5.44% Neutron Fraction
The burner's 5.44% neutron fraction is a specific, honest number that rules out the word aneutronic and keeps shielding and lifetime engineering on the table.
A number with consequences
The burner (Aegis / MetroVolt) design carries a neutron fraction of 5.44% — the share of fusion power that appears as neutrons rather than charged particles. It is low compared with a D-T machine, and it is not zero. That single figure carries several honest consequences we state rather than gloss.
What 5.44% implies
- Neutron shielding is still required — the machine is not aneutronic
- Material activation and neutron-driven wear still occur, at reduced levels
- The large charged-particle majority is what enables direct energy conversion
- Lifetime engineering must account for real, if lower, fluence
Reconciling reported figures
Neutron fraction can be quoted at different points in a design — for instance a design point value versus a figure that includes additional reactions or a different operating assumption. Where two figures appear, we reconcile them in public rather than picking whichever reads better. The frozen design-point value is 5.44%, and any other figure is explained against it.
A precise neutron fraction is more useful to an engineer than a reassuring adjective. 5.44% tells them how much shielding to design; 'aneutronic' would tell them to design none, and be wrong.