Experimental Determination of the Spectral Indices ²⁸ρ and ²⁵δ of the IPEN/MB-01 Reactor

This experimental work aims to obtain the spectral indices ²⁸ρ and ²⁵δ of the core of the IPEN/MB-01 nuclear reactor with a novice approach based on the direct gamma spectrometry of the irradiated fuel rods. The measurements were performed with high precision at the asymptotic central core region. The total and epithermal reaction ratios were obtained from the direct gamma spectrometry of a irradiated fuel element at the maximum neutron flux position. The gamma spectrometry was performed by a fuel rod scanning equipment with a collimator of 1 cm opening size. The data were acquired allowing the irradiation of the fuel rod to be bare or covered with cadmium sleeves of variable sizes. Correction factors were obtained both experimentally and from a calculation approach in order to correct the perturbations due to cadmium. Such calculations, carried out by the MCNP-4B, show the behaviour of these factors as a function of the cadmium sleeve length. The analysis shows that the correction factors are very sensitive to the nuclear data library used by MCNP-4 B; namely ENDF/B-V and ENDF/B-VI.5 libraries. The best results are obtained when the correction factors and the spectral indices are calculated with ENDF/B-VI.