I’m studing air activation around a D-T neutron generator comparing FLUKA and MCNP. Around the source a primary shielding of graphite, high density polyethylene and borated polyethylene is present. I want to evaluate air activation inside and ouside the primary shielding, in the regions called “INPU” and “INSHIEL” in FLUKA simulation (cell 2 and 6 in MCNP simulation).
In both code the photonuclear reactions are not activated. I am aware that Fluka considers the default HYDROGEN material bounded to WATER (not considered in MCNP) but it’s not relevant in this case.
Fluka uses JEFF3.3 cross sections library whereas in MCNP I used ENDF/B-VII.1 cross section (80c: room temperature). The two set of data are quite similar, so I will expect very close results.
I have essentially two questions:
Fluka provides radionuclides that are unexpected, i.e. H-3. In your opinion, what could be the reason?
In MCNP I considered the main air constituent (N-14, O-16 e Ar-40) with the main neutron reaction (n,y), (n,a), (n,p), (n,2n) that could occur. The main discrepancy is in N-13 production as you can see in the following table. Do you have some idea about this discrepancy?
Dear Davide,
thank you for your interest.
In the meantime I want to point out some assumptions that I forgot to say in my first post.
I excluded photonuclear reactions in both codes because I evaluated that they are negligible. For example, the N-13 produced by N-14 (g,n) N-13 is very low.
I didn’t consider the thermal scattering law and free gas treatment in both codes.
Furthermore, I think I solved the problem of the tritium presence in the FLUKA simulation: it comes from the N-14 (n,T) N-3 reaction which I didn’t consider in MCNP simulation.
In addition, I found the occurrence of the following reaction O-16 (p,a) N-13 by the protons induced by the neutrons in the water reactors. Could this reaction be the reason why the FLUKA result of N-13 is an order of magnitude larger than MCNP? In my MCNP simulation protons are not trasported. https://www.osti.gov/etdeweb/servlets/purl/20931006
Thanks for your reply. Yes indeed, with a reasonable approximation, I would also consider negligible the photonuclear contribution to the production of N-13: I had a look at the cross section and the threshold is rather high (10 MeV).
H-3 can be light a fragment produced in spallation reactions so FLUKA scores it as residual nucleus once it stops in a given region. I am not an MCNP expert, but I think that to get a feeling of its production (without necessarily adding it to the particles transported) you could add an additional multiplier with MT=105 for the (n,t) reaction.
Concerning the N-13 production, yes you should account for the proton contribution.
Yes indeed, deuterons are transported. You can find the list of particles transported by FLUKA (unless explicitly discarded) at the manual page: 5.1. Particles codes — FLUKA Manual
