A noise analysis approach for measuring the decay constants and the relative abundance of delayed neutrons in a zero power critical facility

This experimental work introduces a novice approach for the determination of the decay constants and the relative abundance of the delayed neutron. The experimental procedure consists in obtaining the CPSD from the signals of two compensated ionization chambers in the frequency range from 0.005 to 5.0 Hz approximately. Assuming the point kinetic model, the theoretical expression for the CPSD can be written and the experimental CPSD can be fitted employing a least-square approach. The fitting parameters are β_i and λ_i. A preliminary experiment was made in the IPEN/MB-01 reactor. The least-square approach was able to fit all the betas for a six group model. However, it was not possible to fit the first decay constant (the group of longest half-live) accurately although the other five were achieved. The analysis reveals that the relative abundances of ENDF/B-IV and -VI are in a fair good agreement and that it is not only possible to perform successfully such measurements but also the decay constants and the relative abundance can be measured with a very good level of accuracy. ENDF/B-VI heavily underestimates the reactivity for small negative periods because its first decay constant is overestimated. (author)