Operational modal analysis for characterization of mechanical and thermal–hydraulic fluctuations in simulated neutron noise

Abstract

In the study of the neutron noise in KWU pre-Konvoi PWRs, techniques, such as The Hilbert Huang Transform, but mainly traditional Fourier analysis is used widely to infer spectral characteristics, nevertheless these techniques exhibit important limitations in decomposing the signal so that we can distinguish the contribution of different phenomena (thermal-hydraulic and mechanical perturbations) in the same frequency range. Besides, there are difficulties to gather and present all the results in a single plot which shows the response of the core as a whole, making complex the data visualization. To overcome these limitations, an alternative methodology is researched, the so-called, Operational Modal Analysis (OMA), concretely the Frequency Domain Decomposition (FDD). This methodology is widely used in the study of the dynamic properties of systems and structures. The FDD was performed on a series of neutron noise signals from simulated scenarios based on the transient nodal code SIMULATE-3K. The simulations considered assume different sets of perturbation, from individual sources of fluctuations to combined sources. The methodology separates in the two first singular values and singular vectors the responses due to mechanical vibrations and thermal-hydraulic fluctuation in all the frequency range, in such a way that allows distinguishing different phenomena taking place at an equal frequency range as well as the increase of the response due to each phenomenon. The good performance of OMA in the present study provides promising possibilities to infer characteristics of the input excitation from the neutron noise data in PWR. Finally, the methodology shows remarkable advantages in the compilation of the results which can be utilized for monitoring purposes.