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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

The Need for Precise and Well-documented Experimental Data on Prompt Fission Neutron Spectra from Neutron-induced Fission of {sup 239}Pu

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 with 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.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2019

Expansion of the surrogate method to measure the prompt fission neutron multiplicity for Pu 241 [Prompt Fission Neutron Multiplicity for Pu-241 using the Surrogate Method]

Fission neutron multiplicity data are imperative for nuclear models that support criticality safety, nuclear nonproliferation, and stockpile stewardship. This work presents the first experimental measurement to determine the average neutron multiplicity ($\bar{ν}$) and the neutron multiplicity distribution using a surrogate reaction. 241 Pu has a short half-life of 14.3 yr. To circumvent experimental difficulties, inelastic scattering of 53.96-MeV α particles on 242 Pu served as a surrogate for 241 Pu (n , f). The present text reports and discusses the average neutron multiplicity, in addition to the neutron multiplicity moments and distribution for equivalent neutron energies up to 20 MeV. Finally, the $\bar{ν}$ measured in this experiment agrees with previous measurements until 12 MeV, where discrepancies arise due to preequilibrium neutron emission.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2016

Improved fission neutron energy discrimination with 4 He detectors through pulse filtering

Our paper presents experimental and computational techniques implemented for 4 He gas scintillation detectorsfor inducedfission neutron detection. Fission neutrons are produced when natural uranium samples are activelyinterrogated by 2.45 MeV deuterium-deuterium fusion reaction neutrons. Fission neutrons of energies greaterthan 2.45 MeV can be distinguished by their different scintillation pulse height spectra since 4 He detectorsretain incident fast neutron energy information. To enable the preferential detection of fast neutrons up to10 MeV and suppress low-energy event counts, the detector photomultiplier gain is lowered and triggerthreshold is increased. Pile-up and other unreliable events due to the interrogating neutronflux and backgroundradiation arefiltered out prior to the evaluation of pulse height spectra. With these problem-specific calibrationsand data processing, the 4 He detector's accuracy at discriminatingfission neutrons up to 10 MeV is improvedand verified with 252 Cf spontaneousfission neutrons. Given the 4 He detector's ability to differentiate fastneutron sources, this proof-of-concept active-interrogation measurement demonstrates the potential of specialnuclear materials detection using a 4 He fast neutron detection system.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2017

Measurement of prompt fission neutron spectrum for spontaneous fission of Cf 252 using γ multiplicity tagging

The prompt fission neutron spectrum from spontaneous fission of 252 Cf is an integral part of several aspects of nuclear data. Not only is the spectrum itself of interest, but neutron detectors often use the spectrum for calibration, and other prompt fission neutron spectra are measured as a ratio to 252 Cf. Therefore, reducing the uncertainties in this spectrum will allow for more accurate nuclear data to be available across a wide range of fields. The prompt fission neutron spectrum for the spontaneous fission of 252 Cf was measured at Rensselaer Polytechnic Institute using the multiple γ tagging method with a 18.4-ng fission sample. An EJ-301 liquid scintillator fast neutron detector was used to measure the high energy portion of the spectrum, 0.5–7 MeV, and a thin EJ-204 plastic scintillator was used to measure the low energy portion of the spectrum, from 50 keV to 2 MeV. These spectra both show good agreement with the current evaluation of 252 Cf and have low associated uncertainties providing a new high precision measurement that helps reduce the uncertainties in the prompt fission neutron spectrum for the spontaneous fission of 252 Cf.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2018

Correlated Production and Analog Transport of Fission Neutrons and Photons Using Fission Models FREYA, FIFRELIN, and the Monte Carlo Code TRIPOLI-4

Fission modeling in general-purpose Monte Carlo transport codes often relies on average nuclear data provided by the international evaluation libraries. As such, only average fission multiplicities are available and correlations between fission neutrons and photons are missing. Whereas uncorrelated fission physics is usually sufficient for standard reactor core and radiation shielding calculations, correlated fission secondaries are required for specialized nuclear instrumentation and detector modeling. For coincidence counting detector optimization for instance, the precise simulation of fission neutrons and photons that remain correlated in time from birth to detection is essential. New developments were recently integrated into the Monte Carlo transport code TRIPOLI-4 to model fission physics more precisely, the purpose being to access event-by-event fission events from two different fission models: Fission Reaction Event Yield Algorithm (FREYA) and Fission Fragment Evaporation Leading to an Investigation of Nuclear Data (FIFRELIN). TRIPOLI-4 simulations can now be performed, either by connecting via an application programming interface to the Lawrence Livermore National Laboratory fission library including FREYA, or by reading the external fission event data files produced by FIFRELIN beforehand. These new capabilities enable us to easily compare results from the Monte Carlo transport calculations using the two fission models in a nuclear instrumentation application. In the first part of this paper, the broad underlying principles of the two fission models are recalled. We then present the experimental measurements of neutron angular correlations for 252 Cf(sf) and 240 Pu(sf). The correlations were measured for several neutron kinetic energy thresholds. In the latter part of this paper, simulation results are compared with the experimental data. Spontaneous fissions in 252 Cf and 240 Pu are modeled by FREYA or FIFRELIN. Emitted neutrons and photons are subsequently transported to an array of scintillators by TRIPOLI-4 in the analog mode to preserve their correlations. Angular correlations between fission neutrons obtained independently from these TRIPOLI-4 simulations, using either FREYA or FIFRELIN, are compared with the experimental results. In conclusion, for 240 Pu(sf), the measured correlations were used to tune one of the FREYA model parameters. Similarly, different sets of parameters have been tested in FIFRELIN to try to improve the agreement with the experimental data.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2015

Preliminary Evaluation and Uncertainty Quantification of the Prompt Fission Neutron Spectrum of {sup 239}Pu

Low evaluated uncertainties were obtained in a previous evaluation of the {sup 239}Pu prompt fission neutron spectrum and associated covariances for incident neutrons of 0.5 MeV, which were enlarged a-posteriori before being incorporated into ENDF/B-VII.1. These low evaluated uncertainties triggered an in-depth study and improved estimate of experimental as well as model uncertainties. Here, we will summarize these efforts and show that the improved estimate of experimental and model uncertainties leads to corresponding evaluated uncertainties in good agreement with uncertainties obtained in a statistical analysis based primarily on experimental information.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2015

Evaluation of the 239 Pu prompt fission neutron spectrum induced by neutrons of 500 keV and associated covariances

We present evaluations of the prompt fission neutron spectrum (PFNS) of (PU)-P-239 induced by 500 keV neutrons, and associated covariances. In a previous evaluation by Talon et al. (2010), surprisingly low evaluated uncertainties were obtained, partly due to simplifying assumptions in the quantification of uncertainties from experiment and model. Therefore, special emphasis is placed here on a thorough uncertainty quantification of experimental data and of the Los Alamos model predicted values entering the evaluation. In addition, the Los Alamos model was extended and an evaluation technique was employed that takes into account the qualitative differences between normalized model predicted values and experimental shape data These improvements lead to changes in the evaluated PENS and overall larger evaluated uncertainties than in the previous work. However, these evaluated uncertainties are still smaller than those obtained in a statistical analysis using experimental information only, due to strong model correlations. Hence, suggestions to estimate model defect uncertainties are presented. which lead to more reasonable evaluated uncertainties. The calculated k(eff) of selected criticality benchmarks obtained with these new evaluations agree with each other within their uncertainties despite the different approaches to estimate model defect uncertainties. The k(eff) one standard deviations overlap with some of those obtained using ENDF/B-VILl, albeit their mean values are further away from unity. Spectral indexes for the Jezebel critical assembly calculated with the newly evaluated PFNS agree with the experimental data for selected (n,) and (n,f) reactions, and show improvements for highenergy threshold (n,2n) reactions compared to ENDF/B-VII.l. (C) 2015 Elsevier B.V. All rights reserved.

Neudecker, D.↗
DOE OSTI2019

Preequilibrium Asymmetries in the Pu 239 ( n , f ) Prompt Fission Neutron Spectrum

The physical properties of neutrons emitted from neutron-induced fission are fundamental to our understanding of nuclear fission. However, while state-of-the-art fission models still incorporate isotropic fission neutron spectra, it is believed that the preequilibrium prefission component of these spectra is strongly anisotropic. The lack of experimental guidance on this feature has not motivated incorporation of anisotropic neutron spectra in fission models, though any significant anisotropy would impact descriptions of a fissioning system. In the present work, an excess of counts at high energies in the fission neutron spectrum of 239 Pu is clearly observed and identified as an excess of the preequilibrium prefission distribution above the postfission neutron spectrum. This excess is separated from the underlying postfission neutron spectrum, and its angular distribution is determined as a function in incident neutron energy and outgoing neutron detection angle. Comparison with neutron scattering models provides the first experimental evidence that the preequilibrium angular distribution is uncorrelated with the fission axis. The results presented here also impact the interpretation of several influential prompt fission neutron spectrum measurements.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2017

The 235 U prompt fission neutron spectrum measured by the Chi-Nu project at LANSCE

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- 6 Li 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 235 U PFNS.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2025

Re-evaluating the prompt fission neutron spectrum of spontaneously fissioning 252 Cf

The prompt fission neutron spectrum (PFNS) of spontaneously fissioning 252 Cf is a Neutron Data Standards observable. Nearly all fission spectra of actinides were measured relative to it, using efficiencies derived from it, or analyzed with simulations validated by it. The current Standards evaluation was published by W. Mannhart in 1987. It could not be updated because the evaluation input, experimental mean values and covariances, were lost. First, we attempt to reproduce it. However, Mannhart’s evaluation can only be reproduced within its one-σ uncertainties as some of its aspects (e.g., experimental covariances, rejected data points) remain unknown. Therefore, a new evaluation is presented: We revisit all existing experimental 252 Cf(sf) PFNS data, including those published after the release of the current Standards evaluation, and re-estimate associated covariances. The newly evaluated 252 Cf(sf) PFNS differs distinctly from Mannhart’s below 300 keV and extends it to lower and higher outgoing neutron energies (500 eV–25 MeV). The new evaluated uncertainties are larger from 3–9 MeV and smaller otherwise. Spectrum averaged cross sections of importance to the International Reactor Dosimetry and Fusion File community calculated with the new spectrum are close to those calculated with Mannhart’s evaluation and agree with experimental values well within their uncertainties.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2023

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

An evaluation of the average prompt fission neutron multiplicity, $\bar{ν}_p$, of 239 Pu(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 $\bar{ν}_p$ as prior enables to link, through the use of evaluated model input parameters, $\bar{ν}_p$ to other fission observables such as the prompt fission neutron spectrum (PFNS), preneutron 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 $\bar{ν}_p$ agrees well (χ 2 ≈ 1) with data. Moreover, with the new evaluated $\bar{ν}_p$, 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 239 Pu PFNS and fission cross section. Due to these encouraging validation results, the evaluated $\bar{ν}_p$ is currently part of a release candidate for the 239 Pu ENDF/B-VIII.1 file. Hence, a correlated fission model was used for the first time for evaluating $\bar{ν}_p$ that is of evaluation quality. This is an important step towards consistent evaluations of prompt fission observables.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2021

Errors introduced in fission neutron spectrum measurements using a single reference

Measurements of physical quantities are often made relative to a reference quantity, simplifying the interpretation of the measurement and its uncertainties. However, if applied without careful considerations, this technique can carry with it a significant and complex systematic error that is usually ignored. For example, measurements of continuous neutron spectra relative to a reference spectrum yield a result that does not properly include the important effects of neutron scattering in the experimental environment. These effects act to distort the measured spectrum shape and create notable errors in spectrum-integrated quantities. While this effect is one of a series of known potential sources of systematic error including, but not limited to target impurities, angular distributions, incident beam flux, and others, the error introduced from differences in environmental scattering effects between the reference and desired spectra are frequently overlooked. In this work, we demonstrate the origin of this error using simulated data corresponding to measurements of the neutron-induced prompt fission neutron spectra of 233 U, 235 U, 238 U, and 239 Pu as well as the spontaneous fission neutron spectrum of 252 Cf. Errors in the spectrum shape can be as high as 10%–15% depending on the combination of incident and outgoing neutron energy. The spectrum integral and average spectrum energy also contain errors that change with incident neutron energy and can be as high as ~2%. Although this study is based on simulated data, we also show a method for proving that this error exists in an experimental data set and we show a correction that can be applied to results of this type to obtain a more accurate result, thereby largely removing systematic errors from environmental scattering effects. The potential for these kinds of errors should be considered in measurements producing continuous neutron spectra.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2015

Current Issues in Nuclear Data Evaluation Methodology: {sup 235}U Prompt Fission Neutron Spectra and Multiplicity for Thermal Neutrons

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 data 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.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2015

Impact of new prompt fission neutron spectrum evaluations and the associated cross-correlations on critical assemblies using MCNP6 - 14385

Two recent re-evaluations of the neutron-induced prompt fission neutron spectrum (PFNS) of uranium and plutonium actinides have included some cross-correlated uncertainties that have not been released in a new nuclear data library. The first re-evaluation included cross-isotope correlations of the PFNS within each of the uranium and plutonium suites of actinides below the second-chance fission threshold. The second, more recent re-evaluation includes incident energy-dependent correlations of the {sup 239}Pu(n,f) PFNS. Both of these new evaluations and their cross-correlations, given the importance of these actinide quantities and because the impact of the incident energy-dependent correlations has never been studied previously, could have an impact on a variety of criticality safety applications. For example, the ke uncertainty due to the uranium PFNS uncertainties in the Big Ten critical assembly is underpredicted by ∼10% when the cross-isotope correlations in the uranium PFNS are ignored. (authors)

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2018

Chi-Nu measurements of the prompt fission neutron spectrum of {sup 235}U and {sup 239}Pu - 25581

Knowledge of the prompt fission neutron spectrum is crucial for understanding the behavior of energetic nuclear systems. Although this energy spectrum has been measured for the major actinides {sup 235}U and {sup 239}Pu for decades, the majority of the available literature data lack the coverage of both incident and outgoing neutron energies and the precision required to accurately guide evaluations and fission models. The Chi-Nu experiment is designed to measure the prompt fission neutron spectrum for a wide range of incident and outgoing neutron energies with attention to uncertainty estimation and assessment of systematic errors in order to provide reliable guidance for evaluations and models of nuclear fission. Preliminary results of the prompt fission neutron spectrum from the neutron-induced fission of {sup 235}U and {sup 239}Pu are presented here, with a focus on the presence of spectral features near multi-chance fission thresholds, pre-equilibrium neutrons in the deduced spectrum, and also on trends of the mean fission neutron energy as a function of the incident neutron energy. While the mean energies of from ENDF/B-VIII.0 are in good agreement with those reported for {sup 235}U spectra, significant disagreements exist above an incident neutron energy of approximately 13 MeV for {sup 239}Pu. Furthermore, in general, the physics features regarding multi-chance fission and pre-equilibrium neutron emission newly-introduced into ENDF/B-VIII.0 appear provide an improved description of the data as compared to ENDF/B-VII.1. (authors)

61 RADIATION PROTECTION AND DOSIMETRY↗