Variables
| Symbol | Quantity | Unit |
|---|---|---|
| φ | Neutron flux | n/(cm2*s) |
| n | Neutron density | n/cm3 |
| v | Neutron speed | cm/s |
What it means
Neutron flux (φ) is a fundamental quantity in nuclear reactor physics and neutronics. It is defined as the product of the neutron density n (number of neutrons per unit volume) and their average velocity v. Physically, it represents the total path length travelled by all neutrons in a unit volume per unit time, or the number of neutrons crossing a unit area per second. The flux is used to calculate reaction rates: reaction rate per unit volume = Σ φ, where Σ is the macroscopic cross‑section. In a reactor, the flux distribution determines the power distribution and fuel burnup. Neutron flux is measured using neutron detectors and is a key parameter in reactor control and safety. Understanding flux is essential for reactor design, core management, and radiation shielding calculations.
Worked example
Neutron Flux – Two Examples
Real‑World| Parameter | Value |
|---|---|
| n | 1×10⁸ n/cm³ |
| v | 2.2×10⁵ cm/s |
| Parameter | Value |
|---|---|
| n | 5×10⁷ n/cm³ |
| v | 2.2×10⁵ cm/s |
Common mistakes
- Neutron flux φ: The number of neutrons crossing a unit area per unit time – units: n/(cm²·s) or n/(m²·s).
- Neutron density n: The number of neutrons per unit volume – units: n/cm³ or n/m³.
- Neutron speed v: The average speed of the neutrons – in cm/s or m/s.
- Flux vs. fluence: Flux is a rate (per time); fluence is the time integral.
- Energy dependence: Flux is often energy‑dependent – use the appropriate group flux for multi‑group calculations.
Applications
Neutron flux, φ = n·v, is the product of neutron number density and their average speed. It is a measure of the intensity of neutron radiation and is a key parameter in nuclear reactor design and shielding. Reactor physicists use neutron flux to determine the reaction rates, the burnup of fuel, and the activation of structural materials. In research reactors, the flux determines the availability of neutrons for scattering experiments and isotope production. In radiation protection, knowledge of neutron flux is essential for designing shielding and for assessing dose rates to personnel. By accurately determining neutron flux, engineers can optimise reactor operation, ensure safety, and enable advanced neutron‑based research.
- Nuclear reactor core design and neutronics analysis
- Calculation of nuclear reaction rates and fuel consumption
- Design of neutron sources and research reactors
- Radiation shielding design against neutron radiation
- Neutron activation analysis and materials characterisation
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