Release of Evaluated 239Pu(n,f) Prompt Fission Neutron Spectra Including the CEA and Chi-Nu High-precision Experimental Data
Résumé
This report documents an evaluation of 239Pu prompt fission neutron spectra (PFNS) that is
a release candidate for the upcoming U.S. nuclear data library, ENDF/B-VIII.1. This evaluation
differs from its predecessor, ENDF/B-VIII.0, mainly by the inclusion of two 239Pu PFNS high-
precision experiments: one measured by the Chi-Nu team of LANL and LLNL, and the second
one by the CEA team using the liquid-scintillator Chi-Nu array. These two data sets are the first
ones that cover the 239Pu PFNS for continuous incident-neutron energies of 1–20 MeV and broad
outgoing-neutron energies with high precision. Chi-Nu covers outgoing-neutron energies from 10
keV–10 MeV, while CEA data are given for 0.25–11.3175 MeV. Previous data sets were either
measured in a limited energy range or with less precision. Hence, these new CEA and Chi-Nu
data provide decisive information for the evaluation. The resulting evaluated data correspond well
to the new experimental PFNS, but are distinctly different than ENDF/B-VIII.0 values at all
incident-neutron energies. These large differences compared to ENDF/B-VIII.0 can also be seen in
the average mean energies computed from the new evaluated PFNS. However, the evaluated mean
energies agree well with experimental data from Chi-Nu and CEA data, again, indicating that while
the changes are large compared to ENDF/B-VIII.0, they are supported by experimental data. Due
to those differences, large drops (e.g., -128 pcm for Jezebel or -114 for Flattop-Plutonium) in the
predicted effective neutron multiplication factor, keff , of selected ICSBEP critical assemblies can be
observed when these new PFNS are used compared to using ENDF/B-VIII.0. These large changes
can be counter-balanced by considering, in addition to the new 239Pu PFNS, new evaluated data for
the neutron-induced 239Pu fission cross section from the Neutron Data Standards project and the average prompt fission neutron multiplicity from an NCSP project. A further indication that the
evaluated data describe high-precision experimental data well is that the the evaluated PFNS also
produce 239Pu/235U PFNS in agreement with associated Chi-Nu data. Changes in the simulation
of 239Pu LLNL pulsed-sphere neutron-leakage spectra are modest compared to ENDF/B-VIIII.0
PFNS
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