Neutron lifetimes and return fraction experimental determination and analyses in several configurations of the IPEN/MB-01 nuclear reactor and its impact in the determination of the reactivity of the system

This work proposes the development of a new methodology for measurement of the reactivity in the IPEN/MB-01 research reactor facility, based on microscopic and macroscopic noise experiments and the Two-Region point kinetic equations model. Differently from the other point kinetic theoretical models, the Two-Region model assumed in this work takes into account the nuclear reactor how a coupled system, which constitute the theoretical basis of all mathematical development, contemplate both regions of the reactor (core and reflector).The study of the reflector effect and its impact in the reactivity is an original fact and, to make possible the viability of this study, the kinetic parameters related to the reflector must be obtained. The main advantage of this new methodology is to obtain the kinetic parameters from the reflector in a purely experimental way. In order to validate this new method, a series of experiments involving different types of reflectors was performed in the IPEN/MB-01 reactor. The reflectors constituted by Light Water, Stainless Steel (SS-304) and Heavy Water were employed. The Rossi-α neutron noise technique were applied in several subcritical states to obtain the parameters of the reflector. Furthermore, the Auto Power Spectral Densities were also used for a comparison between the experimental data. Moreover, the MCNP-5 nuclear reactor physics code with the ENDF/B-VII.0 library neutron data was employed to calculate the reactivity through the k_eff multiplication factor for each experimental configuration. In this way, from the Two-Region point kinetic equations model were obtained the theoretical expressions in which were used for least squares fit of the experimental data. The neutron lifetimes in the reflector (τr) and in the core (τC), and the neutron return fraction from the reflector to the core (f) were obtained as least squares fitted parameters and then employed for the reactivity calculation through the Inhour two region equation. The presented experimental and theoretical results are referring to the standard core configuration with aforementioned reflectors of Light Water, Stainless Steel (SS-304) and Heavy Water. For all experiments the 26 x 28 fuel rod configuration was employed with the detectors operating in pulse mode. (author)