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Theneutron flux is ascalar quantity used innuclear physics andnuclear reactor physics. It is the total distance travelled by all freeneutrons per unit time and volume.^[1] Equivalently, it can be defined as the number of neutrons travelling through a small sphere of radius${\displaystyle R}$ in a time interval, divided by a maximal cross section of the sphere (thegreat disk area,${\displaystyle \pi R^2}$) and by the duration of the time interval.^[2]: 82-83 Thedimension of neutron flux is${\displaystyle {\mathsf{L}}^{-2}{\mathsf{T}}^{-1}}$ and the usualunit is cm⁻²s⁻¹ (reciprocalsquare centimetre timesreciprocal second).

Theneutron fluence is defined as the neutron fluxintegrated over a certain time period. So its dimension is${\displaystyle {\mathsf{L}}^{-2}}$ and its usual unit is cm⁻² (reciprocal square centimetre). An older term used instead of cm⁻² was "n.v.t." (neutrons, velocity, time).^[3]

Neutron flux inasymptotic giant branchstars and insupernovae is responsible for most of the naturalnucleosynthesis producingelements heavier thaniron. In stars there is a relatively low neutron flux on the order of 10⁵ to 10¹¹ cm⁻² s⁻¹, resulting in nucleosynthesis by thes-process (slow neutron-capture process). By contrast, after a core-collapse supernova, there is an extremely high neutron flux, on the order of 10³² cm⁻² s⁻¹,^[4] resulting in nucleosynthesis by ther-process (rapid neutron-capture process).

Earth atmospheric neutron flux, apparently from thunderstorms, can reach levels of 3·10⁻² to 9·10⁺¹ cm⁻² s⁻¹.^[5][6] However, recent results^[7] (considered invalid by the original investigators^[8]) obtained with unshielded scintillation neutron detectors show a decrease in the neutron flux during thunderstorms. Recent research appears to support lightning generating 10¹³–10¹⁵ neutrons per discharge viaphotonuclear processes.^[9]

Artificial neutron flux refers to neutron flux which is man-made, either as byproducts from weapons or nuclear energy production or for a specific application such as from aresearch reactor or byspallation. A flow of neutrons is often used to initiate thefission of unstable large nuclei. One or more additional neutrons may cause a nucleus to become unstable, causing it to decay (split) to form more stable products. This effect is essential infission reactors andnuclear weapons.

Within a nuclear fission reactor, the neutron flux is the primary quantity measured 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 per unit time.

Areactor vessel of a typical nuclear power plant (PWR) endures in 40 years (32 full reactor years) of operation approximately 6.5×10¹⁹ cm⁻² (E > 1MeV) of neutron fluence.^[10] Neutron flux causes reactor vessels to suffer fromneutron embrittlement and is a major problem with thermonuclear fusion likeITER and other magnetic confinement D-T reactors where fast (originally 14.06 MeV) neutrons damage equipment resulting in short equipment lifetime and huge costs and large volumes of radioactive waste streams.

• Neutron radiation
• Neutron transport

• Neutron
• Physical quantities

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