# Flux in Moderator vs Flux in fuel

1. Dec 5, 2013

### sandon

I was reading about different flux shapes, and it depicted the the shape of the fission neutron spectrum to be in the shape of y=e^-x*sinh(sqrt(x)), where y is the spectrum and x is the energy in MeV.

note: put that equation into wolfram for easy visualization

Then it depicted the slowing down of neutrons neutron flux to be proportional to 1/E

Finally it gave the neutron spectrum of thermal neutron to be proportional to
(E*exp(-E/kt))/T^(3/2)

See attached image for full neutron spectrum

My question is how does the shape of the neutron flux in the moderator differ from the shape of the neutron flux in the fuel.

My first thoughts were neutron flux in the fuel is the full spectrum of the attached image, and the flux in the moderator does not contain as much of the fission spectrum. I cant see any other differences, but the question wants a clear indication of difference between the fuel spectrum and moderator spectrum, both dependent on energy.

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2. Dec 8, 2013

### Astronuc

Staff Emeritus
The fast flux is typically about 3x thermal flux in a thermal reactor.

The fission neutrons are produced with a spectrum. About 8 to 10% of fissions in a thermal reactor occur due to fast fissions of U-238.

The thermal flux in the fuel falls off radially into the fuel due to self-shielding, i.e., the fissions and reasonance absorptions diminish the neutron flux penetrates the fuel.

I will try to find a reference on the neutron spectum in an LWR.

Ref: http://meetingsandconferences.com/physor2012/Workshops/9.SFR-physics.pdf (third page)

Last edited: Dec 21, 2013