Adjustment of JEFF-3.3 data for U-235 and Pu-239 in the fast, non-resonant energy range

A stochastic technique based upon the Serpent code is used along with the JEFF-3.3 nuclear data library consistently including their covariance, to adjust fast, non-resonant ENDF-6 format data for U-235 and Pu-239, and this in conjunction with the Asymptotic Generalized Linear Least-Squares (AGLLS) assimilation methodology. The current investigations are focused on simple, well documented critical systems such as Godiva and Jezebel exhibiting much stronger sensitivities of the assimilated parameters to this nuclear data. Correspondingly, the assimilation database consists of effective multiplication factors along with ratios of fission reaction rates per atom for Pu-239 and U-235 which are spectral indices measured at core center. By not identifying the fission cross-section of U-235 with a standard, we find that this data set should be decreased between 2 keV and 2.2 MeV; in relative terms by maximum 5.9 % occurring in the energy range between 41 keV and 67 keV which is less than two standard deviations (sigma of 6.6 %, obtained from the covariance files of the JEFF-3.3 library. Where as (nu) over bar needs to be increased by up to similar to 0.3 %, corresponding to half a sigma. Also to reduce is the elastic scattering P-1 cross-section between 111 keV and 10 MeV with a stronger decrease amounting to 12.7 % in the range 2.2 MeV-3.7 MeV. For the capture cross-section, we find a decrease of maximum 3 % between 111 keV and 3.7 MeV, in which case the one-sigma limit is about 10 %. For Pu-239, the results of the adjustment study also indicate a reduction of the elastic scattering P-1 cross-section in the same energy range as for U-235, however by only 5.7 %. The fission and inelastic scattering cross-section changes are much smaller than 1 %; as opposed to U-235, (nu) over bar is to decrease, and this by less than 0.5 % corresponding to one sigma. The additionally proposed fission spectrum change not exceeding the one-sigma limit somewhat shifts the median emission energy to a lower value. Differently from U-235, the adjustment of the capture cross-section is an increase of maximum 1.8 % in the range 2.2 MeV3.7 MeV, which is well below the one-r limit of the JEFF-3.3 library. (c) 2022 Elsevier Ltd. All rights reserved.

Automated summary

In this study, a stochastic technique based on the Serpent code is used to adjust the nuclear data for U-235 and Pu-239. The adjustment is done by using the JEFF-3.3 nuclear data library, which includes covariance data, and the Asymptotic Generalized Linear Least-Squares (AGLLS) assimilation methodology. The results show that the adjusted parameters include a decrease in the fission and elastic scattering cross-sections for U-235, an increase in (nu) over bar, and a decrease in the elastic scattering cross-section for Pu-239.