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

Abstract

a b s t r a c tWe present evaluations of the prompt fission neutron spectrum (PFNS) of 239 Pu induced by 500 keV neutrons, and associated covariances. In a previous evaluation by , 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 PFNS 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-VII.1, 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,γ) and (n,f) reactions, and show improvements for highenergy threshold (n,2n) reactions compared to ENDF/B-VII.1.

Evaluating the239Pu Prompt Fission Neutron Spectrum Induced by Thermal to 30 MeV Neutrons

“…Jezebel and Flattop benchmarks are higher than 1 for ENDF/B-VII.1 and are now lower. The k eff values for the suite of seven PSTs is slightly closer to 1 than the respective ENDF/B-VII.1 value, mostly due to new 16 O data. While these results look encouraging, one has to consider that a critical assembly benchmark is not a sufficient validation constraint of evaluated data, as many reactions contribute to calculations of assemblies and biases in several of them might compensate each other.…”

“…They were frist calculated using ENDF/B-VII.1 data, and then with a modified ENDF/B-VII.1 file, which was updated with the current evaluation, the findings of the WPEC SG34 [22] on the neutron multiplicity and CIELO 16 O data [2]. Jezebel and Flattop benchmarks are higher than 1 for ENDF/B-VII.1 and are now lower.…”

“…Compared to the ENDF/B-VII.1 evaluation [3], (1) recently published experimental data [10,11] were implemented, (2) detailed experimental uncertainties were estimated including uncertainties associated with previously unknown possible biases [13,14], (3) the original Los Alamos model [17] was extended [16], (4) the pre-equilibrium component to the PFNS was considered and (5) the incident energy dependence of important model parameters was included in the model description following [18,19]. The evaluated results shown in Fig.…”

“…The pre-equilibrium component was not considered in ENDF/B-VII.1. The variables N i (E) are the spectra of neutrons emitted promptly after fission in first-chance (i = 1), second-chance (i = 2), etc., processes and are calculated by an extended version [16] of the Los Alamos model [17] (LAM), while the exciton model was used for the pre-equilibrium component. The variables P f i yield the probability that fission of i th chance occurs.…”

“…[18,19], while the total kinetic energy was assumed to be constant for the ENDF/B-VII.1 evaluation. Also, uncertainties of more model parameters following [16] were included for this evaluation compared to ENDF/B-VII.1.…”