The burner's neutrons are not all from D-D directly. Deuterium-deuterium reactions breed tritium in the plasma, and about 17.3% of it burns in secondary D-T reactions before it leaves — a neutronic channel the design computes explicitly rather than ignoring.
One branch of the D-D reaction produces tritium, which can then fuse with deuterium in a 14 MeV neutron reaction. The burner computes this secondary burnup from the deuterium density, reactivity, and particle confinement time, finding 17.3% burnup at the reference point — a real contribution to the neutron budget, and one that grows with confinement.
Accounting for the secondary burn is why the burner's neutron fraction is computed from the full reaction network, not from the D-3He channel alone. It is also the mechanism behind the confinement-neutron coupling: better confinement burns more of the bred tritium.
| Bred-tritium burnup | 17.3% (reference) |
| Channel | secondary D-T (14 MeV) |
| Source | D-D breeding branch |
| Computed from | n_D, reactivity, confinement time |
| Grows with | confinement |