Deuterium–helium-3 fusion releases its energy overwhelmingly in charged particles. In MetroVolt's staged, catalysed cycle, only 5.25% of fusion power leaves as neutrons — a design constant that quietly rewrites the entire plant.
D-T fusion — the mainstream choice — emits 80% of its energy as 14.1 MeV neutrons, which demand a meter-scale breeding blanket, activate the structure, and set the replacement clock for every component behind the wall. D-³He's primary reaction is neutron-free; the residual neutron budget comes from unavoidable D-D side reactions, and MetroVolt's staged fuel management holds it to a computed 5.25% neutronicity at the 80:20 operating mix.
That single number cascades: first-wall load 0.10–0.12 MW/m² (roughly an order of magnitude below a D-T plant), vessel dose ~1.0 dpa per full-power year, and — because there is no tritium fuel to breed — no breeding blanket at all.
Low-neutron is a maintenance strategy, a licensing posture, and a cost line, all at once. The deposited low-neutron dividend study (S81) prices it: a comparable D-T plant faces ~20–25 blanket changeouts over 30 full-power years, capping its availability near 0.71–0.75 — a +33–42% levelized-cost penalty MetroVolt structurally never pays.
| Neutronicity f_n | 5.25% (80:20 mix) |
| First-wall load | 0.10–0.12 MW/m² |
| Vessel dose rate | ≈1.0 dpa / full-power year |
| Breeding blanket | none required |
| Avoided D-T penalty | +33–42% LCOE (S81, derived) |