Representation of the Fission Spectrum and Multiplication Calculations
We investigated the different representations of the fission process as total fission on one hand and as the sum of direct fission, first chance fission, second, third and fourth chance fission on the other hand (MT = 18 versus MT = 19, 20, 21 and 38 in ENDF terminology).
The representations of these cross sections (MF = 3) in ENDF/B libraries are generally consistent, but the fission spectrum (MF=5) of the total fission given in libraries is generally not equivalent to the fission spectrum obtained from all partial spectra.
We calculated the effective multiplication factor k eff of several simple near critical systems and the neutron leakage from a central 14 MeV neutrons source embedded in simple subcritical systems. We found that in certain cases there is a non-negligible difference in the results, depending on the representation of the fission spectrum that was used; in other cases the differences were small. These results are explained in the paper.
As a conclusion we propose improvements in the methodology of the representation of fission in ENDF-6 format and also suggest changes in the data of some materials in ENDF/B-VI.
KeywordsFission spectrum ENDF/B-VI direct fission first chance fission
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