Units and meaning of detector output (mean flux values)

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Chris Forsyth
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Units and meaning of detector output (mean flux values)

Post by Chris Forsyth » Thu May 27, 2021 5:08 am

Hello,

I'm trying to calculate the average neutron flux (neutrons / cm^2 * s) across my entire geometry, in each energy bin.

Using the same syntax as the tutorial on the Wiki, I have:

det EnergyDetector de MyEnergyGrid
ene MyEnergyGrid 3 500 1E-11 2E1

First of all, does this ^ detector automatically take tallies over the entire geometry (inside the system boundaries)?

Here is one of the rows in my resulting detector output file:

417 417 1 1 1 1 1 1 1 1 1.57914E+00 0.00281

What are the units of the bolded ^ quantity above? Does it represent the average neutron flux (neutrons / cm^2 * s) in energy bin 417, across the entire system volume? This value seems very low for a neutron flux (although it is for a zero-power reactor, and I suppose the neutron flux will be much lower in the large reflectors outside the core).

What type of postprocessing do I need to do to get it in the desired form? Do I need to divide by the total system volume...and do I need to do anything with the energy bin width? What changes to my input syntax would you recommend?

Thanks!

Ville Valtavirta
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Re: Units and meaning of detector output (mean flux values)

Post by Ville Valtavirta » Thu May 27, 2021 8:53 am

Hi,

since you are not defining any spatial integration limits, your detector will tally over the whole geometry.

In order to get an idea of the units for detector output see first the beginning of Chapter 7 in the Serpent manual.

Basically, detectors will integrate the chosen response over their spatial domain and divide the result with the user given detector volume or if user has not given a detector volume, by 1.

So your detector outputs (spatially integrated scalar flux [cm/s]) / (detector volume = 1.0 cm^3) = [1/cm^2s]. However, in order to truly get the correct average scalar flux you'll need to either set the detector volume (dv) or divide by the correct detector volume by hand.

The actual numerical value for the flux seems low, have you set the simulation normalization to some specific power level?

-Ville

Chris Forsyth
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Re: Units and meaning of detector output (mean flux values)

Post by Chris Forsyth » Thu May 27, 2021 9:09 am

Thank you Ville, this was very helpful.

You're right, I didn't set a specific power level. It makes sense that this is the issue; since it's only a simulation of a few thousand particle histories, of course the flux levels would be low. I guess I'll normalize all of them using the maximum flux value and represent my flux-energy spectrum that way!

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