Producing Evaluation -quality 239Pu Average Prompt Fission Neutron Multiplicities using a Correlated Fission Model

An evaluation of the average prompt fission neutron multiplicity, Pp, of Pu-239(n,f) is shown. This evaluation includes (a) the correlated fission model CGMF, and (b) a detailed analysis of past and recently published experimental data. Using CGMF-calculated Pp as prior enables to link, through the use of evaluated model input parameters, Pp to other fission observables such as the prompt fission neutron spectrum (PFNS), pre neutron emission fission yields as a function of mass, and the average total kinetic energy of the fragments. These evaluated parameters produce realistic predictions of many fission observables, while the evaluated (v) over barp agrees well (chi(2) approximate to 1) with data. Moreover, with the new evaluated Pp, the effective neutron multiplication factor of fast Pu ICSBEP critical assemblies are predicted with a mean bias of 58 pcm compared to 18 pcm with ENDF/B-VIII.0, when paired with a new (PU)-P-239 PFNS and fission cross section. Due to these encouraging validation results, the evaluated Pp is currently part of a release candidate for the Pu-239 ENDF/B-VIIL 1 file. Hence, a correlated fission model was used for the first time for evaluating Pp that is of evaluation quality. This is an important step towards consistent evaluations of prompt fission observables.