From: Feracci Vincent (email@example.com)
Date: Wed Dec 05 2007 - 11:38:14 CET
I sent a message some time ago concerning trouble with fission product
yield. The new FLUKA patch completely fixed the issue (so thank you
for this release!). However, I still would like to know how FLUKA
determines the fission product yield : in my case, U238 fission by
FLUKA seems to use EndfB-VI, Jeff and Jendl nuclear data tables to
determine fission yields. However, these tables only have fission
yield values for 500 keV and 14 MeV neutrons concerning U-238. But if
I use 40 MeV neutrons, I still have yet a different fission yield
compared to 14 MeV neutrons. So I suppose FLUKA uses physic models for
high energy neutrons.
Is there any documentation available on the models used for fission
product yields concerning fissions induced by high energy neutrons?
> I am currently simulating a U238 target bombarded by high energy neutrons
> (~1 MeV->40 MeV, mean energy ~ 12 MeV) and I am interested in the
> fission yield of U238. I compare the results with MCNPX coupled with
> DARWIN. However, the results I obtain aren't exactly what I expected :
> - In terms of fission rate, I have almost the same results with MCNPX
> and FLUKA
> (~ 6.E+13 fission/s)
> - In terms of fission yield there are two differences, one in amount :
> Using MCNPX with DARWIN, I find that 1,1E+14 fission fragments (I
> considered elements with 25<Z<67) are generated per seconds which
> seems to agree with a 6.E+13 fission/s fission rate.
> On the other hand, using the RESNUCLEI card in FLUKA, I find that only
> 3,4E+13 fission fragments are generated per second. This value seems
> to be 3 or 4 times too low. Is it just that there is obviously
> something wrong with my input? (I have put my input in attachment plus
> the source routine I use)
> However RESNUCLEI tells me that a great quantity of U238 is produced
> in inelastic interactions( ~ 1.4E14 nuclide/s). Does this mean that
> some of these interactions are fissions but RESNUCLEI only gives info
> on an intermediate state?
> ( supposing the U238 nuclide didn't break immediately but is excited
> and will break shortly but the fission products won't be accounted for
> in RESNUCLEI).
> My other concern is the shape of my fission fragment spectrum :
> I have compared the normalized fission fragment spectrum obtained by
> using FLUKA with the results obtained using MCNPX with DARWIN and by
> using ENDFBVI fission yield values with the neutron fluence in the
> target calculated by MCNPX.
> The fission fragment spectrum obtained using MCNPX and DARWIN seems to
> underestimate the formation of mid-sized fragments (110<A<125)
> probably because it doesn't use tables of high energy neutron fission
> yields (See attached graph).
> The use of ENDFB-VI tables with values for 14 MeV neutron fissions
> still gives me a difference of a factor 3 to 4 in the fission yield
> for mid-sized fragments in between the FLUKA results and the ones
> obtained using the ENDFB-VI tables.
> In the manual it is said that fission fragments are determined using
> evaluated data extracted from European, American and Japanese
> databases. Could this difference come from the use of different
> Or does Fluka use physic models after 14 MeV to determine fission
> fragments yield?
> This could explain the difference in the spectra as around 16 percent
> of the neutrons which reach the target have an energy included in
> between 20 and 40 MeV.
> So to sum up a bit my questions, If I have around 6.E+13 fissions per
> second and only 3,4E+13 fission fragments produced per second
> according to RESNUCLEI, is there definitely something wrong with my
> input or is there a reasonable explanation?
> What tables does FLUKA use to determine the fission yield spectrum?
> Does FLUKA use physic models for high energy neutrons?
> Vincent Feracci
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