Release of Evaluated 235U(n,f) Average Prompt Fission Neutron Multiplicities Including the CGMF Model
This report documents an evaluation of the average prompt fission neutron multiplicity, $$\overline{v}_p$$, of 235U from 200 keV to 15 MeV that is a potential release candidate for the upcoming U.S. nuclear data library, ENDF/B-VIII.1. This evaluation had to be re-done from "scratch", as the input to the $$\overline{v}_p$$ evaluation of the previous library, ENDF/B-VIII.0, was lost. That means that all available experimental data were re-analyzed and uncertainties were re-estimated. Another major difference to ENDF/B-VIII.0 is that this evaluation includes model information from the Hauser-Feshbach fission fragment decay code CGMF, while ENDF/B-VIII.0 is based purely on experimental data. CGMF links several fission quantities with each other; $$\overline{v}_p$$ is predicted by assumptions made on, e.g., pre-neutron emission yields as a function of mass, the total kinetic energy, or spin and parity of fission fragments. This allows to perform two types of validation for the new 235U $$\overline{v}_p$$: On the one hand, one can employ evaluated CGMF parameters obtained from fitting to experimental 235U $$\overline{v}_p$$ to predict yields as a function of mass, the average total kinetic energy, or the mean energy of the prompt fission neutron spectrum. These model-predicted values can then be compared to experimental and evaluated data. The model-predicted fission-observable values using evaluated parameters obtained here are reasonably close to experimental data indicating the evaluated 235U(n,f) $$\overline{v}_p$$ are physical. On the other hand, one can validate 235U $$\overline{v}_p$$ with respect to integral responses such as fast ICSBEP critical assemblies or LLNL pulsed spheres. LLNL pulsed-sphere neutron-leakage spectra are minimally impacted by the new 235U $$\overline{v}_p$$ as these experimental data are shape data and the $$\overline{v}_p$$ would mostly lead to a change in normalization of the data as the spheres are relatively thin (0.7 and 1.5 mean-free path) and, thus, mostly depend on 235U $$\overline{v}_p$$ from 12-15 MeV. The change in the predicted effective neutron multiplication factor, keff, of selected ICSBEP critical assemblies, however, is large compared to values using ENDF/B-VIII.0 and experimental keff: The average bias is 108 pcm across all studied keff values versus 12 pcm for ENDF/B-VIII.0. A reasonable performance in simulating keff (mean bias of 14 pcm) can be retained by tweaking 235U $$\overline{v}_p$$ from 3-5 MeV, and combining it with a recent 235U PFNS evaluation that is also a ENDF/B-VIII.1 release candidate.
- Research Organization:
- Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)
- Sponsoring Organization:
- USDOE National Nuclear Security Administration (NNSA), Nuclear Criticality Safety Program (NCSP)
- DOE Contract Number:
- 89233218CNA000001
- OSTI ID:
- 1863730
- Report Number(s):
- LA-UR-22-23475; TRN: US2308267
- Country of Publication:
- United States
- Language:
- English
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