The burner's D-3He cycle sends 5.44% of its power out as neutrons at the locked design point. That is roughly an order of magnitude below a D-T plant — genuinely low-neutron — but it is not zero, and Kronos never calls it aneutronic. The mandatory descriptor is low-neutron; the word matters as much as the number.
D-3He's primary reaction is neutron-free, but unavoidable deuterium-deuterium side reactions and a secondary D-T burn leave a residual neutron budget: a frozen 5.44% of fusion power at the design point (helium-3 fraction 0.30). Running richer cleans it further — 4.18% at x=0.35 — but true near-aneutronic operation (x ≥ 0.45) breaks closure and needs a different machine. 'Aneutronic' would imply zero; the real figure is small, finite, and engineered for.
Overclaiming 'aneutronic' is a credibility trap the field has fallen into before. By fixing the mandatory term at 'low-neutron' and printing the percentage, the burner's shielding, activation, and maintenance stories all rest on the true number rather than on a marketing word.
| Neutron fraction (design point) | 5.44% of fusion power (M-43 restated) |
| At x=0.35 clean-shift | 4.18% |
| Versus D-T | ~order of magnitude lower |
| Mandatory descriptor | low-neutron (never aneutronic) |
| Near-aneutronic (x >= 0.45) | needs a redesigned machine |