No Chain Reaction, No Criticality
Fusion has no neutron multiplication feedback, so the concept of criticality — central to fission safety — does not apply.
In fission, a neutron splits a nucleus and releases more neutrons, which split more nuclei: a chain reaction. Safety is defined around criticality — keeping the neutron multiplication factor at or below one so the chain neither grows nor requires constant suppression. Reactivity accidents are, at root, criticality excursions.
Fusion has no such loop
Fusion energy comes from fusing light nuclei, and the neutrons it releases do not go on to cause more fusion. There is no neutron-multiplication feedback, no critical mass, and no configuration in which the reaction becomes self-amplifying. You cannot make a fusion plasma ‘go critical’ because criticality is not a property it can have.
The breeder (Hyperion) does use its 14 MeV neutrons to breed tritium and helium-3 in a surrounding blanket, and a neutron multiplier can increase the tritium breeding ratio (studied across 1.1 / 1.5 / 1.8). But multiplication in the blanket is a fixed material property that boosts neutron yield modestly and is not a feedback loop on the fusion rate — it cannot run away.
- No critical mass and no criticality condition exist for a fusion plasma.
- Neutrons from fusion do not trigger more fusion.
- Blanket neutron multiplication is a bounded material effect, not a chain reaction.
This removes an entire class of reactivity accidents from the safety analysis. See no fissile material and proliferation resistance.