Why D–Helium-3 Over D–Tritium
D–He-3 trades a far harder ignition condition for a mostly charged energy release and a much lower neutron flux on structures.
A deliberate trade
D–T is the easiest fusion fuel: its cross-section peaks near 60–70 keV and it ignites at the lowest triple product of any candidate. But four-fifths of D–T energy leaves as a 14.1 MeV neutron, activating structures and demanding a thermal blanket. The burner accepts a harder physics problem to avoid that.
The reward is physical, not economic: the main D–3He channel is charged, so it can be collected directly and the structural neutron load falls by more than an order of magnitude relative to D–T. The residual neutrons come from unavoidable D–D side reactions.
The penalties are real
D–3He requires a higher ion temperature (~90 keV vs ~15–20 keV for D–T ignition studies), a higher triple product, and a fuel — helium-3 — that is scarce at terrestrial scale. These are exactly the levers behind the burner's honest gates, and they are the reason the breeder (Hyperion) exists: it bakes helium-3 into the program's fuel loop.
The comparison is not that one fuel is better absolutely, but that each suits a different job. D–T suits a machine whose purpose is to breed fuel and make products near-term; D–3He suits a resilient direct-conversion generator whose purpose is firm, low-activation power. The program runs both because the D–T breeder is what makes the helium-3 the D–3He burner needs — the two fuels are links in one chain, not rivals.
- D–T: easy ignition, ~80% neutron energy
- D–3He: hard ignition, ~5.44% neutrons, charged output
- Trade favors direct conversion and low activation
- Penalties: temperature, triple product, He-3 scarcity