Trouble with fission yield

From: Feracci Vincent (
Date: Fri Nov 09 2007 - 16:40:49 CET

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    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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