The experimental determination of the 238U(n,γ) and total fission reaction rates along the pellet radius of the IPEN/MB-01 reactor

Abstract

This work presents a new experimental approach to determine experimentally the reaction rate along the pellet radius of a research reactor facility. The methodology employs concentric hollow cylindrical lead collimators together with gamma-ray spectrometry in a special kind of HPGe detector for the discrimination of very low gamma energy emitted by 239Np and 99Mo. The 238U neutron capture and total fission rates were inferred, respectively, from the 106.2keV gamma emitted by 239Np and from the 140.51keV gamma-ray emitted by 99Mo. These lower gamma-ray energies allow the thickness of the lead collimator to be small enough so that the correction factors applied to the procedure were minimized. This kind somewhat challenging experiment was successfully performed at the IPEN/MB-01 research reactor facility. The experiments are claimed to be well-defined, and they are suitable for a benchmark. The measured values of the total fission rates are mainly due to the thermal fissions in 235U since in the IPEN/MB-01 reactor nearly 85% of the total fissions occur in the thermal neutron energy region. The theoretical analyses were performed using MCNP-5 together with the ENDF/B-VII.0 library. The analyses reveal a very good agreement between the calculated and experimental results for the 238U epithermal neutron capture reaction rates. However, the same can not be said for the thermal reaction rates which show discrepancies both in magnitude as well as in the shape of the attenuation of the reaction rates inside of the fuel pellet. The suspected reason for these discrepancies is the shape of the 235U cross sections below 0.3eV which might be different from that adopted in the ENDF/B-VII.0 library.