Finalizing the ⁶³Cu and ⁶⁵Cu Resonance Evaluations for the ENDF/B-VIII Release

The resolved resonance region evaluation for the only two natural isotopes of copper (⁶³Cu and ⁶⁵Cu) was carried out at the Oak Ridge National Laboratory (ORNL) in support of the US Department of Energy (DOE) Nuclear Criticality Safety Program (NCSP). The particular interest in the isotope of copper arose because it was linked with poor performance of the Zeus series of integral benchmarks from the International Criticality Safety Benchmark Evaluation Project (ICSBEP). Good agreement between the calculated and experimentally measured values for the ICSBEP benchmarks is essential to the NCSP because the ICSBEP benchmarks are used for validation purposes of criticality safety calculations. The particular set of four Zeus experiments had a highly-enriched uranium core with a thick copper-reflector resulting in a large sensitivity of the criticality of these systems to the cross section of copper in the intermediate energy range. Sobes et al. presented the effect of the newly resolved resonance region evaluations for the two isotopes of copper on the Zeus integral benchmarks, as well as 23 other benchmarks from the ICSBEP during the 2016 ANS Annual Meeting. The results presented in Ref. were calculated with the ENDF/B-VII.1 cross section libraries used for all other materials. However, it is suggested that the resolved resonance region evaluations for the isotopes of copper should not be judged only on their performance in the integral benchmark calculations especially when the ENDF/B-VII.1 cross sections are used for all of the other isotopes. Three reasons are provided for this. First and foremost, a new evaluation is an assessment of the physics of the nucleus of the particular isotope, and the goal of an application agnostic nuclear data evaluation is to improve evaluation by fitting state-of-the art R-Matrix formalism to highly precise experimental cross section measurements. The improved performance in the integral benchmarks should only come as a consequence of the improvement in the physics of the evaluation. Secondly, there is some uncertainty in the nuclear data evaluation of the cross section of all isotopes, so there could be some error in evaluating another isotope used in calculating the integral benchmark experiment. Such an error may have been having a compensating effect, in combination with the previous evaluations for the isotopes of copper. Thirdly, with the two isotopes of copper proposed for inclusion in the upcoming release of the ENDF/B-VIII library, performance of the new resonance evaluation for copper should be evaluated based on the calculation of the integral benchmarks with all of the evaluations proposed for inclusion in the ENDF/B-VIII release. These benchmarking results using the ENDF/B-VIII evaluations as they converge closer to their final form will be presented at the conference. This summary stresses the importance of the physics of the evaluation and reviews some of the steps taken in finalizing a resolved resonance region evaluation. (authors)