Measurement of the Energy Spectrum of the Neutrons inside the Neutron Flux Trap Assembled in the Center of the Reactor Core IPEN/MB-01

This paper presents the neutron energy spectrum in the central position of a neutron flux trap assembled in the core center of the research nuclear reactor IPEN/MB-01, obtained by an unfolding method. To this end, we have used several different types of activation foils (Au, Sc, Ti, Ni, and plates) which have been irradiated in the central position of the reactor core (setting number 203) at a reactor power level (64.57 ± 2.91 watts). The activation foils were counted by solid-state detector HPGe (high pure germanium detector) (gamma spectrometry). The experimental data of nuclear reaction rates (saturated activity per target nucleus) and a neutron spectrum estimated by a reactor physics computer code are the main input data to get the most suitable neutron spectrum in the irradiation position obtained through SANDBP (spectrum analysis neutron detection code-version Budapest University) code: a neutron spectra unfolding code that uses an iterative adjustment method. The adjustment resulted in (3.85 ± 0.14) × 10 9 n·cm -2 ·s -1 for the integral neutron flux, (2.41 ± 0.01) × 10 9 n·cm -2 ·s -1 for the thermal neutron flux, (1.09 ± 0.02) × 10 9 n·cm -2 ·s -1 for intermediate neutron flux and (3.41 ± 0.02) × 10 8 n·cm -2 ·s -1 for the fast neutrons flux. These results can be used to verify and validate the nuclear reactor codes and its associated nuclear data libraries, besides, show how much effective it can be that the use of a neutron flux trap in the nuclear reactor core to increase the thermal neutron flux without increase the operation reactor power level. The thermal neutral flux increased 4.04 ± 0.21 times compared with the standard configuration of the reactor core.