Measurement of Leakage Neutron Spectra from a Spherical Pile of Niobium Bombarded with 14MeV Neutrons and Validation of Its Nuclear Data

Abstract

For validation of nuclear data files for Nb, the absolute leakage neutron spectrum from a spherical pile of Nb was measured in the energy range between 0.1 and 16MeV at OKTAVIAN. A sensitivity analysis was performed with MCNP-4A and the modified JENDL-3.2 libraries. The measured spectrum was compared with the MCNP-4A calculation with two nuclear data libraries processed from JENDL-3.2 and ENDF/B-VI. The comparison was made by the spectrum shape and the C/E values in four energy regions. The JENDL-3.2 calculation gives satisfactory prediction except below 0.8 MeV, but that with ENDF/B-VI considerably overestimates the spectrum above 1 MeV. From the observation of the calculated and measured spectra and of the secondary energy distribution spectra (SED) of the (n, 2n) and inelastic scattering in JENDL-3.2 and ENDF/B-VI, the discrepancies in the calculations are probably due to improper SED of the (n, 2n) reaction for that below 0.8 MeV in JENDL-3.2 and that between 0.8 and 6 MeV and SED of inelastic scattering for the secondary neutron energy between 5 and 10 MeV in ENDF/B-VI. Therefore, the (n, 2n) reaction values in JENDL-3.2 library was reduced by 20% and the SED below 0.8 MeV was replaced with that of ENDF/B-VI, and this library reproduced the experiment within about 10% discrepancy over all energy regions.