The fuel-cleanliness trajectory.
How clean the burner runs is a choice, not a constant. Adding helium-3 to the mix cuts the neutron fraction roughly fivefold across the closing window — for a price in fuel demand.
- Lever
- x(³He) — the helium-3 fraction of the fuel
- Closing window
- x(³He) ∈ [0.20, ~0.43]
- Neutron fraction across it
- ≈ 13.0% → 2.5% (~5×)
- Frozen point
- x = 0.30 → f_n 5.44%
- Free clean-shift
- x 0.30 → 0.35 → f_n 4.18%, still net-positive
Every D–³He plasma also runs some D–D side reactions, and those produce neutrons. The fraction of power carried by neutrons, f_n, is therefore set by how much helium-3 is in the mix: more helium-3 crowds out the neutron-producing channels. Across the burner's closing window, f_n falls from about 13% at the lean edge to about 2.5% at the rich edge — a roughly fivefold swing.
| x(³He) | neutron fraction f_n | engineering gain Q_E |
|---|---|---|
| 0.20 (lean edge) | ≈ 9.5% | 1.13 |
| 0.25 | ≈ 7.1% | 1.27 |
| 0.30 (frozen) | 5.44% | 1.31 |
| 0.35 | ≈ 4.2% | 1.24 |
| 0.40 | ≈ 3.2% | 1.12 |
The frozen operating point sits at x = 0.30, where the machine both closes strongly (Q_E 1.31) and runs clean (f_n 5.44%). There is a "free clean-shift" available: moving to x = 0.35 cuts the neutron fraction to 4.18% while staying net-positive. Pushing toward truly aneutronic operation (x ≥ 0.45) does not close on this configuration — that would be a redesign, not a fuel change.
This is why Kronos calls the fuel low-neutron, not aneutronic. The neutron fraction is small and adjustable, but it is not zero, and the record says so.