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Title: Evaluation of the 235U prompt fission neutron spectrum including a detailed analysis of experimental data and improved model information

Abstract

We present an evaluation of the 235U prompt fission neutron spectrum (PFNS) induced by thermal to 20-MeV neutrons. Experimental data and associated covariances were analyzed in detail. The incident energy dependence of the PFNS was modeled with an extended Los Alamos model combined with the Hauser-Feshbach and the exciton models. These models describe prompt fission, pre-fission compound nucleus and pre-equilibrium neutron emissions. The evaluated PFNS agree well with the experimental data included in this evaluation, preliminary data of the LANL and LLNL Chi-Nu measurement and recent evaluations by Capote et al. and Rising et al. However, they are softer than the ENDF/B-VII.1 (VII.1) and JENDL-4.0 PFNS for incident neutron energies up to 2 MeV. Simulated effective multiplication factors k eff of the Godiva and Flattop-25 critical assemblies are further from the measured k eff if the current data are used within VII.1 compared to using only VII.1 data. However, if this work is used with ENDF/B-VIII.0β2 data, simulated values of k eff agree well with the measured ones.

Authors:
ORCiD logo [1];  [1];  [1]; ORCiD logo [1]; ORCiD logo [1]
  1. Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
Publication Date:
Research Org.:
Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1396117
Report Number(s):
LA-UR-16-27468
Journal ID: ISSN 2100-014X
Grant/Contract Number:
AC52-06NA25396
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
EPJ Web of Conferences
Additional Journal Information:
Journal Volume: 146; Conference: ND2016 ; 2016-09-11 - 2016-09-11 ; Brugge, Belgium; Journal ID: ISSN 2100-014X
Publisher:
EDP Sciences
Country of Publication:
United States
Language:
English
Subject:
73 NUCLEAR PHYSICS AND RADIATION PHYSICS

Citation Formats

Neudecker, Denise, Talou, Patrick, Kahler, Albert Comstock III, White, Morgan Curtis, and Kawano, Toshihiko. Evaluation of the 235U prompt fission neutron spectrum including a detailed analysis of experimental data and improved model information. United States: N. p., 2017. Web. doi:10.1051/epjconf/201714604012.
Neudecker, Denise, Talou, Patrick, Kahler, Albert Comstock III, White, Morgan Curtis, & Kawano, Toshihiko. Evaluation of the 235U prompt fission neutron spectrum including a detailed analysis of experimental data and improved model information. United States. doi:10.1051/epjconf/201714604012.
Neudecker, Denise, Talou, Patrick, Kahler, Albert Comstock III, White, Morgan Curtis, and Kawano, Toshihiko. Wed . "Evaluation of the 235U prompt fission neutron spectrum including a detailed analysis of experimental data and improved model information". United States. doi:10.1051/epjconf/201714604012. https://www.osti.gov/servlets/purl/1396117.
@article{osti_1396117,
title = {Evaluation of the 235U prompt fission neutron spectrum including a detailed analysis of experimental data and improved model information},
author = {Neudecker, Denise and Talou, Patrick and Kahler, Albert Comstock III and White, Morgan Curtis and Kawano, Toshihiko},
abstractNote = {We present an evaluation of the 235U prompt fission neutron spectrum (PFNS) induced by thermal to 20-MeV neutrons. Experimental data and associated covariances were analyzed in detail. The incident energy dependence of the PFNS was modeled with an extended Los Alamos model combined with the Hauser-Feshbach and the exciton models. These models describe prompt fission, pre-fission compound nucleus and pre-equilibrium neutron emissions. The evaluated PFNS agree well with the experimental data included in this evaluation, preliminary data of the LANL and LLNL Chi-Nu measurement and recent evaluations by Capote et al. and Rising et al. However, they are softer than the ENDF/B-VII.1 (VII.1) and JENDL-4.0 PFNS for incident neutron energies up to 2 MeV. Simulated effective multiplication factors keff of the Godiva and Flattop-25 critical assemblies are further from the measured keff if the current data are used within VII.1 compared to using only VII.1 data. However, if this work is used with ENDF/B-VIII.0β2 data, simulated values of keff agree well with the measured ones.},
doi = {10.1051/epjconf/201714604012},
journal = {EPJ Web of Conferences},
number = ,
volume = 146,
place = {United States},
year = {Wed Sep 13 00:00:00 EDT 2017},
month = {Wed Sep 13 00:00:00 EDT 2017}
}

Journal Article:
Free Publicly Available Full Text
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  • Issues in evaluation methodology of the prompt fission neutron spectra (PFNS) and neutron multiplicity for the thermal-neutron-induced fission of the {sup 235}U are discussed. The inconsistency between the experimental differential and integral data is addressed. By using differential data as ”shape data” good consistency was achieved between available sets of differential data. Integral dosimetry data have been used to define the PFNS slope at high outgoing neutron energies, where the quality of the differential data is poor. The inclusion into the fit of measured integral (spectrum-averaged) cross sections had a very small impact in the region where differential PFNS datamore » are abundant and accurate, but removed the discrepancy with integral data at higher neutron emission energies. All experimental data are consistently fitted giving a PFNS average energy of 2.008 MeV. The impact on criticality prediction of the newly evaluated PFNS was tested. The highly enriched {sup 235}U solution assemblies with high leakage HEU-SOL-THERM-001 and HEU-SOL-THERM-009 benchmarks are the most sensitive to the PFNS. Criticality calculations for those solutions show a significant increase in reactivity if the average neutron energy of the fission neutrons is reduced from the ENDF/B-VI.5 value of 2.03 MeV. The proposed reduction of the PFNS average energy by 1.1% can be compensated by reducing the average number of neutrons per fission ν{sup ¯} at the thermal energy to the Gwin et al. measured value. The simple least-squares PFNS fit was confirmed by a more sophisticated combined fit of differential PFNS data for {sup 233,235}U, {sup 239}Pu and {sup 252}Cf nuclides with the generalised least-squares method using the GMA and GANDR codes.« less
  • The prompt fission neutron spectrum resulting from /sup 239/Pu fission induced by --0.55-MeV incident neutrons is measured from 1.0 to 10.0 MeV relative to that of /sup 235/U fission induced by the same incident energy neutrons. The measurements employ the time-of-flight technique. Energy-dependent ratios of the two spectra are deduced from the measured values. The experimentally derived ratio results are compared with those calculated from ENDF/B-V (revision 2) and with results of recent microscopic measurements. Using the ENDF/B-V /sup 235/U Watt parameters for the /sup 235/U spectrum, the experimental measurements imply a ratio of average fission spectrum energies of /supmore » 239/Pu//sup 235/U = 1.045 +- 0.003, compared to the value 1.046 calculated from ENDF/B-V (revision 2).« less
  • The Chi-Nu experiment aims to accurately measure the prompt fission neutron spectrum for the major actinides. At the Los Alamos Neutron Science Center (LANSCE), fission can be induced with neutrons ranging from 0.7 MeV and above. Using a two arm time-of-flight (TOF) technique, the fission neutrons are measured in one of two arrays: a 22- 6Li glass array for lower energies, or a 54-liquid scintillator array for outgoing energies of 0.5 MeV and greater. Presented here are the collaboration's preliminary efforts at measuring the 235U PFNS.