D–D Side Reactions
Deuterium–deuterium reactions in the hot plasma make neutrons, tritium, and helium-3; they are the source of the burner's 5.44% neutron fraction.
The burner burns deuterium with helium-3, but the plasma is full of deuterium, and deuterium fuses with itself. The D–D reaction has two branches of near-equal probability: one produces a neutron plus helium-3, the other a proton plus tritium. The tritium can then burn with deuterium (D–T), which is strongly neutronic. Together these side channels produce the neutrons the machine must shield against.
This is why D–³He is called low-neutron rather than aneutronic. The 5.44% neutron fraction is the sum of D–D neutrons and secondary D–T neutrons at the burner's density and near-90 keV temperature. Higher deuterium fraction or temperature would raise the D–D rate, so the fuel mix is a lever on neutron output as well as on power.
Products and their handling
- Neutrons — absorbed by shielding, deposited as heat
- In-situ helium-3 — a minor internal source, mostly external fuel
- Tritium — a small inventory that must be managed and can burn
- Protons and helium-4 — charged, captured by direct conversion
Why it is unavoidable
As long as the fuel contains deuterium — and D–³He requires it — D–D reactions occur. They cannot be turned off, only minimized through the fuel mix and operating point. The burner accounts for them honestly: the 5.44% fraction, the shielding it drives, and the small tritium inventory it creates are all consequences of this physics, reported rather than assumed away.
All figures are design-and-simulation values for a machine not yet built.