Radionuclide inventory and Activation evolution under neutron irradiation

Dear FLUKA experts,

Greetings from China!

I am a graduate student, Xianpeng Yin working on the FLUKA for about one year. Currently, I am calculating the radionuclide inventory and time-dependent activation evolution of the naturally abundant pure iron under a typical fusion reactor neutron spectrum. I have a few concerns:

  1. About the source beam definition: If I need to input a text-format neutron spectrum file to define the incident neutron energy distribution, is it appropriate to use only an empty SOURCE card after successfully compiling the executable file in Compile? I am not quite sure whether the format of the energy spectrum file (.txt) I input meets the specifications or rules of FLUKA. (A .txt file has been provided below, where the first column is the lower limit of energy range in MeV, the second column is the upper limit of energy range, and the third column is the neutron flux fluence.)

  2. About the low energy neutron reactions: Since the energy range of the aforementioned neutron spectrum file is between 1E-11 MeV to 20 MeV, does FLUKA have an energy threshold X for neutron-nucleus reactions, or it only depends on the range of nuclear dataset? I used the LOW-PWAS card to activate the point-wise mode. When the energy is lower than X, there is almost no production of radioactive nuclides, and thus there is no subsequent radioactive decay. Is this correct? During the process of radioactive nuclide production, does FLUKA include all possible reactions such as elastic scattering, inelastic scattering, radiative capture, emission of charged particles, and heavy nucleus fission in its calculation scope? Or does it select important neutron reactions for calculation?

  3. About the target geometry setting: Now, I would like to reproduce a benchmark study, as the attached the reference paper. During the “play” with FLUKA and flair, I found that the volume setting of the target material and the grid density setting of usrbin greatly affect the activity (ATOMASS scoring, Bq/g) calculation results. This may be due to FLUKA calculating the average values within the “BIN.” However, when I set the thickness of the target as 0.00438cm, there is only one radionuclide produced. Therefore, my question is, how should I set the target geometry in FLUKA simulation, so that I could make a reasonable comparison with such 0-deminension, inventory calculation code, such as FISPACT?

Lastly, could you help me to check the below input file? Dose it fully or correctly represents the below irradiation information. Do I need to activate other cards to calculate all possible reaction types between iron nuclei and neutrons?

Here are the key input parameters and files:

Material: Naturally abundant pure iron

Target: Thin slice with a side length of 1 cm and thickness of 0.0438 cm

Beam: Neutron spectrum of a typical fusion reactor, coinciding with the target in terms of cross-section

Irradiation time: Continuous 2 years, with a flux: 5.0422E14/cm2s

Shutdown time: Starting from the shutdown initiation.

Thank you very much for your kind guidance! Happy new year in advance!

acti_exer_12.28.flair (3.1 KB)
acti_exer_12.28.inp (2.5 KB)
DEMO.txt (16.8 KB)
source_newgen_neutron_DEMO.f (18.9 KB)

Dear @YinXianpeng ,

Hopefully these answers will put you in a good direction:

  1. Since you are using the source_newgen.f routine you need to make sure that the formatting is ok to be read in the proper way by the sample_histogram_momentum_energy function. To me it seems that the formatting of the file is ok and also the function is called in the proper way. What you could do is use the debugging option of the routine by setting the debug_logical_flag variable at the end of the routine to “.true.” . If you are not sure about the values in the histogram you can also check this post.
  2. I assume here what you varied in terms of “energy threshold X” is the neutron beam energy? When the energy falls below the transport threshold it can subsequently not interact and radionuclides can indeed not be produced. In your input file you use the PRECISIO DEFAULTS which determines physics settings such as particle production and transport thresholds, in this case for neutrons it sets a transport threshold to 1E-5 eV. Please also look in the manual the description on low energy neutron pointwise transport. Here you will find the processes you mention are taking into account. What you do need to worry about in your simulation is to make sure the particle production and transport thresholds are properly set and the needed physics models are taken into account.
  3. You are fully correct that the results will depend on the USRBIN settings since indeed one mesh bin yields an averaged value over the bin volume. Concerning the thickness of the target, low radionuclide yield and comparison with 0D code: here it might be useful to make use of the LAM-BIAS card to artificially increase the inelastic interaction yield. However, since this is a fairly advanced topic I would advise to have a look at this FLUKA basic course lecture on biasing, specifically from slide 16 on.

On this forum, we are not supposed to validate your input or judge if your input files are appropriate to a specific problem. You can check your input iteratively and when results are not as you expected we can help on specific points. I can only advise you to also check the basic course lecture on activation and the corresponding exercise for more general guidance and tips on simulation settings.

Hope this helps and Happy New Year!


Dear Andreas Waets,

I would like to take this opportunity to express my sincere gratitude for your valuable guidance. I feel that I have gained a clear understanding of the key areas of research and a direction to proceed in.

I plan to spend some time studying the content you recommended. If I encounter any difficulties, I will return to the forum for assistance. Additionally, you have helped me realize the importance of independently solving problems. I have been honoured to learn so much from the forum, and I am looking forward to FLUKA flourishing and making more significant impact.

Thank you again for your guidance and support.

Best Regards,
Yin Xianpeng

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