Deuterium–helium-3 fusion releases its energy overwhelmingly in charged particles. In MetroVolt's staged, catalysed cycle, only 7.70% of fusion power leaves as neutrons at the operating baseline — 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 7.70% neutronicity at the 80:20 operating mix (10.20% on the ³He-banking leg; the retired hot-ion evaluation carried 5.25–7.05%).
That single number cascades: first-wall load 0.074–0.089 MW/m² (roughly an order of magnitude below a D-T plant), vessel dose ~0.74 dpa per full-power year (22 dpa over 30 years, under the ~36 dpa qualification), 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 | 7.70% (80:20 baseline; 10.20% banking leg) |
| First-wall load | 0.074–0.089 MW/m² |
| Vessel dose rate | ≈0.74 dpa / full-power year |
| Breeding blanket | none required |
| Avoided D-T penalty | +33–42% LCOE (S81, derived) |