Neutron Fraction and Shielding
The burner's 5.44% neutron fraction is small but not zero, so a fixed installation still needs shielding, activation management, and a controlled radiation boundary.
Small, not zero
D–3He is often called aneutronic, but the studied burner has a 5.44% neutron fraction — the fraction of fusion energy carried by neutrons, arising from D–D side reactions and secondary processes. Small compared with a D–T machine's ~80%, but real, and enough to require shielding and activation control.
What the fraction means on-site
- A shielding boundary is needed around the machine to protect personnel.
- Structural materials will activate over time and must be managed.
- Shielding mass affects siting, hardening, and maintenance access.
Why it still favours a fixed installation
A 5.44% neutron fraction is far more manageable than a full D–T flux, which is one reason the burner is studied for installations near people. The shielding is modest relative to a D–T plant, but it is not negligible, and a fixed emplacement gives room to place it properly.
Honesty here matters: describing the burner as neutron-free would be false. The correct statement is that its neutron output is small and manageable with conventional shielding practice, and that this manageability is part of why a D–3He burner is a candidate for fixed defense power at all.
Manageable is not the same as absent
Calling D–3He aneutronic overstates it; 5.44% of the fusion energy is carried by neutrons from side reactions. That is small enough to manage with conventional shielding and far below a D–T machine, which is part of why the burner is a candidate for power near people, but it is not zero and it drives real shielding, activation, and maintenance requirements.