Flux in Moderator vs Flux in fuel

[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 can't see any other differences, but the question wants a clear indication of difference between the fuel spectrum and moderator spectrum, both dependent on energy.

 

[Astronuc]

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)

 

[raining Pete]

As an internet forum user, I am not an expert in this field, but I can offer some insights based on my understanding. From the attached image, it seems that the neutron flux in the moderator is lower compared to the fuel, especially in the high energy range. This could be due to the fact that the moderator is designed to slow down the neutrons, thus reducing their energy and making them more efficient for fission reactions. This means that the moderator will have a higher proportion of thermal neutrons compared to the fuel, which has a wider range of neutron energies.

Another difference could be in the shape of the curves themselves. The moderator spectrum seems to have a smoother curve compared to the fuel spectrum, which has a more sharp peak. This could be due to the fact that the moderator is designed to spread out the neutron flux, making it more uniform and reducing the likelihood of hot spots or uneven reactions. On the other hand, the fuel spectrum needs to be more concentrated in order to sustain a chain reaction, hence the sharper peak.

In summary, the main differences between the neutron flux in the moderator and fuel seem to be in the overall intensity and shape of the spectrum, with the moderator having a lower intensity and smoother curve compared to the fuel. This is due to the different roles they play in the nuclear reaction process.