Spectral indices of the IPEN/MB-01 reactor: A nuclear data validation

New experimental results for the spectral indices of the IPEN/MB-01 reactor are presented in this work. The experimental approach considers a novice technique for the ²⁸ρ case which does not require any sort of calculated correction factors. This aspect gave to the IPEN/MB-01 experiment an excellent quality and free of possible bias due to these calculated correction factors. The uncertainty analysis show, even considering the uncertainties of the geometric and material data of the facility, that the final uncertainties are small enough and well understood for a benchmark problem. The theory/experiment comparison reveals that there is considerable progress in the ²³⁸U nuclear data for application in thermal reactors. The theory/experiment comparison reveals an excellent agreement for the spectral index ²⁸ρ*; aspect never found before in several other comparisons. The long term overprediction of the ²³⁸U neutron epithermal capture appears to be more related to the deficiencies in the previous experiments generally used to check the ²³⁸U data than to a nuclear data problem. The experimental performed at the IPEN/MB-01 reactor supports the changes in the ²³⁸U nuclear data incorporated in ENDF/B-VII.0 and in the other libraries studied in this work. The theory/experiment comparison of ²⁵δ* and (C8/F) _ept. show that these spectral indices are in general slightly overpredicted, thus suggesting that the thermal fission cross section of ²³⁵U might be a little bit underestimated. The overall analysis of the theory/experiment comparisons show the excellent applicability of ENDF/B-VII.0 for thermal reactors fuelled with slightly enriched uranium.