skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

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. 2017. "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 = 2017,
month = 9
}

Journal Article:
Free Publicly Available Full Text
Publisher's Version of Record

Save / Share:
  • This paper reports that recent measurements define the prompt fission neutron spectrum of {sup 252}Cf from 25 keV to 20 MeV with sufficient accuracy to show that the standard representation by a Maxwell spectrum with the temperature parameter T{sub M} = 1.42 MeV is not adequate. The next simplest macroscopic representation, a Watt spectrum with the two parameters T{sub W} = 1.18 MeV and E{sub W} = 0.36 MeV, fits the recent data surprisingly well, at least as well as the best currently available microscopic models of fission neutron emission. The resulting chi-square does not indicate any need for amore » more sophisticated description, nor is the fit improved by refinements such as superposition of two Watt distributions (for a representative fragment pair), or relativistic corrections.« less
  • The spectrum of neutrons emitted promptly after 239Pu(n,f)—a so-called prompt fission neutron spectrum (PFNS)—is a quantity of high interest, for instance, for reactor physics and global security. However, there are only few experimental data sets available that are suitable for evaluations. In addition, some of those data sets differ by more than their 1-σ uncertainty boundaries. We present the results of MCNP studies indicating that these differences are partly caused by underestimated multiple scattering contributions, over-corrected background, and inconsistent deconvolution methods. A detailed uncertainty quantification for suitable experimental data was undertaken including these effects, and test-evaluations were performed with themore » improved uncertainty information. The test-evaluations illustrate that the inadequately estimated effects and detailed uncertainty quantification have an impact on the evaluated PFNS and associated uncertainties as well as the neutron multiplicity of selected critical assemblies. A summary of data and documentation needs to improve the quality of the experimental database is provided based on the results of simulations and test-evaluations. Furthermore, given the possibly substantial distortion of the PFNS by multiple scattering and background effects, special care should be taken to reduce these effects in future measurements, e.g., by measuring the 239Pu PFNS as a ratio to either the 235U or 252Cf PFNS.« less
  • The spectrum of neutrons emitted promptly after {sup 239}Pu(n,f)—a so-called prompt fission neutron spectrum (PFNS)—is a quantity of high interest, for instance, for reactor physics and global security. However, there are only few experimental data sets available that are suitable for evaluations. In addition, some of those data sets differ by more than their 1-σ uncertainty boundaries. We present the results of MCNP studies indicating that these differences are partly caused by underestimated multiple scattering contributions, over-corrected background, and inconsistent deconvolution methods. A detailed uncertainty quantification for suitable experimental data was undertaken including these effects, and test-evaluations were performed withmore » the improved uncertainty information. The test-evaluations illustrate that the inadequately estimated effects and detailed uncertainty quantification have an impact on the evaluated PFNS and associated uncertainties as well as the neutron multiplicity of selected critical assemblies. A summary of data and documentation needs to improve the quality of the experimental database is provided based on the results of simulations and test-evaluations. Given the possibly substantial distortion of the PFNS by multiple scattering and background effects, special care should be taken to reduce these effects in future measurements, e.g., by measuring the {sup 239}Pu PFNS as a ratio to either the {sup 235}U or {sup 252}Cf PFNS.« less
  • The prompt fission neutron spectrum emitted by a sample of /sup 235/U irradiated with 0.53-MeV neutrons has been measured in the 0.6- to 15-MeV energy range by using time-of-flight (TOF) techniques. In the present work, a major effort was made to obtain an accurate experimental determination of the energy response and efficiency function of the neutron detector over the entire neutron energy range of interest. For this purpose, the TOF spectrometer was calibrated with respect to energy in the 0.5- to 21-MeV range by observing neutron groups from various nuclear reactions. The energy dependence of the neutron detector efficiency wasmore » determined by observing the angular distributions of the H(n,n)H process in the 1- to 15-MeV energy range. The overlapping 0.6- to 3-MeV energy range was covered by the T(p,n)/sup 3/He reaction. The result of the fission neutron spectrum measurements has been used to find a suitable distribution function describing the data in the entire energy interval. The best description was obtained with the distribution N/sub 1/(E) exp(-1.02E)sinh(2.32E)/sup /sup 1///sub 2//. 6 figures, 4 tables, 39 references.« less