17 Neutron diffusion
Understanding the physical process of and deriving neutron diffusion is crucial to understanding higher level concepts that will be used to determine critical core size, volume, and mass.
Neutron diffusion is assumed to follow Fick’s law because just about every physical diffusion process does –
[latex]J_i = -D\frac{d\phi}{di}[/latex]
The diffusion coefficient [latex]D[/latex] in nuclear engineering is called the diffusion length because the units of [latex]D[/latex] are in length here, where as in something like environmental fate and transport, [latex]D[/latex] is typically in units of [latex]L^2/T[/latex]. This is because the flux [latex]\phi[/latex] here is in units of [latex]n/cm^2/s[/latex]. However, the actual process of neutron diffusion is the same as any other diffusion.
In nuclear engineering [latex]J[/latex] is called current.
Fick’s law is not valid for neutron diffusion if –
- The medium is strongly sorbing.
- There are more than three mean free paths from the source to the medium surface.
- Scattering is anisotropic.
Additional notes
