n+²³⁵U resonance parameters and neutron multiplicities in the energy region below 100 eV

Abstract

Abstract. In August 2016, following the recent effort within the Collaborative International Evaluated Library Organization (CIELO) pilot project to improve the neutron cross sections of 235 U, Oak Ridge National Laboratory (ORNL) collaborated with the International Atomic Energy Agency (IAEA) to release a resonance parameter evaluation. This evaluation restores the performance of the evaluated cross sections for the thermaland above-thermal-solution benchmarks on the basis of newly evaluated thermal neutron constants (TNCs) and thermal prompt fission neutron spectra (PFNS). Performed with support from the US Nuclear Criticality Safety Program (NCSP) in an effort to provide the highest fidelity general purpose nuclear database for nuclear criticality applications, the resonance parameter evaluation was submitted as an ENDF-compatible file to be part of the next release of the ENDF/B-VIII.0 nuclear data library. The resonance parameter evaluation methodology used the Reich-Moore approximation of the R-matrix formalism implemented in the code SAMMY to fit the available time-of-flight (TOF) measured data for the thermal induced cross section of n+ 235 U up to 100 eV. While maintaining reasonably good agreement with the experimental data, the validation analysis focused on restoring the benchmark performance for 235 U solutions by combining changes to the resonance parameters and to the prompt resonanceν below 100 eV.

Neutron Multiplicity Correlations with Fission Fragment Mass and Energy from 239Pu(n,f)

“…The fluctuations have been shown to impact nuclear reactor benchmarks by reducing the criticality [4]. A new evaluation of the prompt fission neutron spectrum (PFNS) in the thermal energy range has determined a lower value of the average neutron energy than that reported in the existing evaluated nuclear data libraries [5]. The lower average neutron energy is in agreement with independent evaluations [6].…”