The intensity of neutron radiation, measured as the number of neutrons crossing a unit area of the medium in a unit time. In a nuclear reactor, the neutron flux is used to measure the intensity of the ongoing fission reaction inside. Neutron flux monitoring in a reactor is important because it also indicates the dose received by reactor components, such as the reactor vessel. Collisions with neutrons can change the structure of the material and cause so-called radiative embrittlement. The average neutron flux in a uranium-loaded reactor core is about 3 × 10¹³ neutrons × cm⁻² × s⁻¹.

In a thermonuclear fusion reactor, the neutron flux will be probably ten times higher than in a fission reactor, which places high demands on the material of the vacuum chamber.