Evaluation of Neutron Reactions for ENDF/B-VII: 232–241U and 239Pu

We have completed new evaluations for several actinides in the ENDF/B-VII database and have extensively modified existing evaluations for several other actinides. We compare calculations using the new evaluations with critical assembly reaction rate measurements made at Los Alamos over the past 50 years, covering a range of reactions and differing neutron spectrum and fluence environments. The evaluations are for all neutron-induced reactions with uranium isotopes in the mass range A=232–241 as well as 239Pu, and mostly cover the incident neutron energy range from keV energies to 30 MeV. We combined the results of these analyses with new evaluations of the resolved and/or unresolved resonance regions from Oak Ridge for 233,235,238U and with modified ENDF/B-VI resonance evaluations for the other actinides to produce new neutron-induced evaluations spanning the incident neutron energy range from 10−5 eV to 20 or 30 MeV. Major aspects of this analysis are: systematic accumulation of all relevant experimental data; re-normalization of the neutron data to modern standards; assessment of the applicability of several recent optical model potentials for actinide calculations; interpretation of the experimental results in terms of nuclear theory to allow interpolation and extrapolation of the data into unmeasured regions; and finally, assembly of the experimental and theoretical results into formal evaluated nuclear data files that can be processed for use in applied nuclear programs. In this report we discuss the theoretical analysis and evaluation of all the evaluations, with emphasis on incident neutron energies in the range 10 keV to 20 MeV. We present detailed comparisons of critical assembly simulations with the measurements and include tables of the experimental results. The critical assembly measurements include reaction rates for (n,f), (n,γ), and (n,2n) reactions obtained in the Godiva, Jezebel, Topsy, Bigten, and Flattop assemblies. The evaluations described here are on file at the National Nuclear Data Center at Brookhaven National Laboratory in the ENDF/B-VII database.

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