Re: Trouble with fission yield

From: Feracci Vincent (
Date: Wed Dec 05 2007 - 11:38:14 CET

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    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?


    Vincent Feracci


    > Hi,
    > 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
    > databases?
    > 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?
    > Regards,
    > Vincent Feracci

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