The neutron flux is a quantity used in nuclear reactorphysics corresponding to the total length travelled by all neutrons per unit time and volume, or nearly equivalently number of neutrons travelling through a unit area in unit time. The neutron fluence is defined as the neutron flux integrated over a certain time period.
Neutron flux in asymptotic giant branchstars and in supernova is responsible for most of the natural nucleosynthesis producing elements heavier than iron. In stars there is a relatively low neutron flux on the order of 105 to 1011 neutrons per cm2 per second, resulting in nucleosynthesis by the s-process (slow-neutron-capture-process). By contrast, after a core-collapse supernova, there is an extremely high neutron flux, on the order of 1022 neutrons per cm2 per second, resulting in nucleosynthesis by the r-process (rapid-neutron-capture-process).
Atmospheric neutron flux, apparently from thunderstorms, can reach levels of 3•102 to 5•102 neutrons per cm2 per sec. However, recent results obtained with unshielded scintillation neutron detectors actually show a decrease in the neutron flux during thunderstorms.
Artificial neutron flux refers to neutron flux which is man-made, either as byproducts from weapons or nuclear energy production or for specific application such as from a research reactor or by spallation. A flow of neutrons is often used to initiate the fission of unstable large nuclei. The additional neutron(s) may cause the nucleus to become unstable, causing it to decay (split) to form more stable products. This effect is essential in fission reactors and nuclear weapons.
Within a nuclear fission reactor the neutron flux is primarily the form of measurement used to control the reaction inside. The flux shape is the term applied to the density or relative strength of the flux as it moves around the reactor. Typically the strongest neutron flux occurs in the middle of the reactor core, becoming lower toward the edges. The higher the neutron flux the greater the chance of a nuclear reaction occurring as there are more neutrons going through an area.
A reactor vessel of a typical nuclear power plant (PWR) endures in 40 years (32 full reactor years) of operation approximately 3.5×1019 n/cm² (E>1MeV). Neutron flux causes reactor vessels to suffer from embrittlement and the steel gets activated.