Simultaneous evaluation of uranium and plutonium fast neutron fission cross sections up to 200 MeV for JENDL-5 and its updates

Otsuka, Naohiko*; Iwamoto, Osamu  

The fast neutron fission cross sections of U and Pu were evaluated for the JENDL-5 library up to 200 MeV. The experimental fission cross sections and their ratios in the EXFOR library were reviewed with the source articles. Additionally, Poenitz's data compiled in his GMA database were reviewed. We found about 0 datasets are archived with the uncertainty information sufficient for covariance matrix construction and converted them from EXFOR to an experimental database with their covariance matrices. When the uncertainty information in the source article is missing in the EXFOR entry, we updated the EXFOR entry. We minimized corrections to the experimental database to make our evaluation becomes traceable. The least-squares fitting was performed to the logarithms of the cross sections and their rations in the experimental database by using the simultaneous least-squares fitting code SOK. The best precisions of the group-wise cross sections were achieved around 2 to 3 MeV, where the external uncertainties are 1% (U), 1.5% (U, Pu) or 2.5% (Pu). Well documented experimental fission cross sections are desired for U between 100 and 300 keV and Pu in the whole energy range since the numbers of the usable experimental datasets are limited and it makes the uncertainties in our evaluation relatively high. The evaluated cross sections were also validated against the spectrum averaged fission cross sections measured under the Cf and - neutron fields, and we found the newly evaluated cross sections are consistent with the experimental spectrum averaged cross sections well except for U fission cross sections measured in the Cf neutron fields, which is underestimated by the newly evaluated cross sections as well as those in many other data libraries.