neutron flux is the intensity of neutron flow through a surface — the number of neutrons crossing a unit area each second — the quantity that sets the shielding requirements and material damage rates of a fusion machine.
Neutron flux is commonly confused with neutron fluence, its time-integrated near-neighbor: flux measures the instantaneous rate of neutron passage, typically quoted in neutrons per square centimeter per second, while fluence accumulates that rate over an exposure history. The distinction matters in practice — a component's heating and dose rates at any moment follow the flux, while its total end-of-life damage follows the fluence.
In a 📝fusion power plant burning the 📝D-T reaction, about 80 percent of the energy released leaves as 14 MeV neutrons. Carrying no electric charge, they ignore the magnetic field that confines the 📝plasma and stream outward through the 📝first wall, so the neutron flux at each surface is fixed by geometry and fusion power rather than by anything the field can do. That flux is simultaneously the plant's product and its principal burden: it delivers the heat a blanket converts to electricity and breeds the plant's fuel from lithium, while also driving the shielding thickness needed to protect components such as the 📝HTS magnets, the activation of the structure, and the rate at which materials degrade. Flux at the wall is often re-expressed as neutron wall loading, the neutron power per square meter of first-wall surface — a standard figure of merit for how hard a fusion machine works its materials.