Different energy deposition using LOW-PWXS and group wise cross section

Dear experts,

I’m bombarding beryllium with heavy ion 7Li, using the secondary neutrons to split 235U,and use SCORE to get fission number and energy depositon in regions. I have known that the scoring for fission processes are not yet fully implemented using the pointwise treatment of neutrons from the topic Query regarding ‘score’ card and activation of fission phenomena only - Scoring - FLUKA User Forum (cern.ch). But when I use group wise cross section, the energy deposition is two or three times smaller than the results using point wise.
I wonder if group wise undervalue the 235U fission cross section? If I want to get correct results, should I use LOW-PWXS and mgdraw.f?

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Hi @lichen
Indeed there is no dedicated SCORE counter to get the number of fission events. On the fluka output there is a line describing the total percentage of fission events from low energy neutrons

 Number of secondaries created by low energy neutron per beam particle:
 Prompt radiation      Radioactive decays
   2.2972E+01 (100.%)   0.0000E+00 (100.%) 
   6.7437E+00 (29.4%)   0.0000E+00 ( 0.0%) elastic NEUTRON 
   1.9305E+00 ( 8.4%)   0.0000E+00 ( 0.0%) inelastic NEUTRON 
   5.1783E+00 (22.5%)   0.0000E+00 ( 0.0%) fission NEUTRON 

For more accurate counting, it can be performed with the use of the MGDRAW routine, after each interaction to inspect if there are at least 2 heavy fragments in the stack (FHEAVY).

For 235U I would not expect any major difference in the neutron cross between group wise and point wise.
I made a simple test of a U235 sphere starting with a few keV neutrons inside and did not observe any big difference in the energy deposition between the GW and PW (actually 5% lower in the PW than the GW). Could you provide your input file to see what is the problem?

Hello, @vasilis

Thank you for your detailed reply!

  1. I noticed something very strange, I set up four layers of fuel rods, when only the first layer of fuel rods are uranium, the other layers are set to graphite material, there is a small difference in the energy deposition between the GW and PW. But when the four layers of fuel rods are all uranium, the energy deposition using GW is two or three times smaller than the results using point wise, as I said last time.
    It looks like too much uranium could be causing the wrong results.

  2. I want to get the number of fissions per primary, the FISSIONS Star Density is not the data I need?

  3. I wrote a mgdraw.f, please check it, when NPHEAV≥2, the count +1, I simulated 100 ions, the number of fissions is 8520, then the number of fissions per primary is 85.2,do I understand it right?

Here are my input and mgdraw.f.
0.25AGeV-C.inp (15.6 KB)
0.25AGeV-C001_fission.txt (732.2 KB)
mgdraw.f (11.5 KB)

Hi @lichen,

I was checking your input and it seems you are dealing with a system that is close to criticality. When you approach criticality the energy deposition will exponentially increase tending to become infinite for keff=1.

As a consequence any tiny differences in the cross sections would exponentially amplify the effect. Which is your case, the more Uranium rods you put in your system the closer you approach to criticality and the calculation quickly diverges.

I was comparing the cross sections between GW (based on old DB libraries) and PW (newest evaluations) and indeed they have some small differences. In your case not only the U-235 cross sections are important but also the cross section of all other elements, since they will change slightly the moderation of the neutrons, thus altering the neutron fluence.

Please note that FLUKA (by construction) is not able to perform criticality calculations.

Hello @vasilis ,

Thank for your detailed reply again!

I will use other software to solve this problem. But I want to know about the second and third questions I mentioned last time, maybe I’ll work on another issue later. Thank you!

Hi @lichen
2. For the moment there is no flag to identify the low-energy neutron interactions. So the STARs do not work as you correctly mentioned.
3. The only way to identify the neutron interaction is to inspect the stack(s) as you are doing

  • if there are two fragments in the heavy stack then it is fission
  • only gammas in the genstk and one recoil → capture

  • I would also filter with the ICODE.eq.300, which tells that the call is coming from a neutron interaction (look the comments on the USDRAW)

@vasilis Thank you for your patient response! Best wishes!