Evaluation of 238U neutron cross-sections for the ENDF/B file

Neutron cross-section data for 238U between 0.00001 eV and 15 MeV, available through August 1969, have been evaluated in order to revise the 238U file in the United States ENDF/B data library. Neutron data reevaluated are σt, σγ, σf, σnn'. σne, σn,2n, σn,3n and ν-bar. Emphasis in this paper is placed on the capture, fission and inelastic cross-section evaluations and existing experimental discrepancies. Resolved resonance parameters were re-evaluated to include more p-wave resonances and recent radiation width measurements which lead to an average radiation width of 0.0235 eV for this evaluation. Measurements reported since the previous evaluation have resulted in the following qualitative changes to the data relative to the previous ENDF/B 238U evaluation: the capture cross-section is lower by 5 - 15% between 1 and 100 keV, the fission cross-section is consistently lower by 4-8%, the inelastic scattering cross-section, particularly for excited levels above 0.15 MeV, is lower by 5-15% between 1.3 and 2 MeV, and the total cross-section differs by up to ± 10% above 100 keV. Uncertainties of at least 10% still exist in the capture cross-section below 150 keV. While the 238U/235U fission ratio is reasonably well known, uncertainties in the absolute 238U fission cross-sections are relatively large owing to discrepancies in the 235U fission cross-section above 2 MeV. Large discrepancies in the 238U data have not been adequately resolved by recent measurements and the existing uncertainties, particularly for the capture cross-section, are probably the most important data problem for fast-reactor applications at the present time. (author)