Release of Evaluated 239Pu(n,f) Prompt Fission Neutron Spectra Including the CEA and Chi-Nu High-precision Experimental Data

This report documents an evaluation of 239Pu prompt fission neutron spectra (PFNS) that isa release candidate for the upcoming U.S. nuclear data library, ENDF/B-VIII.1. This evaluationdiffers 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 secondone by the CEA team using the liquid-scintillator Chi-Nu array. These two data sets are the firstones that cover the 239Pu PFNS for continuous incident-neutron energies of 1–20 MeV and broadoutgoing-neutron energies with high precision. Chi-Nu covers outgoing-neutron energies from 10keV–10 MeV, while CEA data are given for 0.25–11.3175 MeV. Previous data sets were eithermeasured in a limited energy range or with less precision. Hence, these new CEA and Chi-Nudata provide decisive information for the evaluation. The resulting evaluated data correspond wellto the new experimental PFNS, but are distinctly different than ENDF/B-VIII.0 values at allincident-neutron energies. These large differences compared to ENDF/B-VIII.0 can also be seen inthe average mean energies computed from the new evaluated PFNS. However, the evaluated meanenergies agree well with experimental data from Chi-Nu and CEA data, again, indicating that whilethe changes are large compared to ENDF/B-VIII.0, they are supported by experimental data. Dueto those differences, large drops (e.g., -128 pcm for Jezebel or -114 for Flattop-Plutonium) in thepredicted effective neutron multiplication factor, keff , of selected ICSBEP critical assemblies can beobserved when these new PFNS are used compared to using ENDF/B-VIII.0. These large changescan be counter-balanced by considering, in addition to the new 239Pu PFNS, new evaluated data forthe 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 theevaluated data describe high-precision experimental data well is that the the evaluated PFNS alsoproduce 239Pu/235U PFNS in agreement with associated Chi-Nu data. Changes in the simulationof 239Pu LLNL pulsed-sphere neutron-leakage spectra are modest compared to ENDF/B-VIIII.0PFN