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

Abstract

To overcome these limitations, in this work, the Frequency Domain Decomposition (FDD) of the Operational Modal Analysis (OMA) is applied to the spectral characterization of the neutron noise.In the study of the neutron noise in KWU pre-Konvoi PWRs, the Hilbert Huang Transform and the widely-used Fourier analysis are employed to extract spectral characteristics. However, both techniques are limited in decomposing the signal in the same frequency range to distinguish the contributions from different phenomena, such as the thermal–hydraulic and the mechanical perturbations. In addition, it is difficult for these two techniques to gather and present all the results in a single plot that shows the response of the core as a whole. To overcome these limitations, in this work, the Frequency Domain Decomposition (FDD) of the Operational Modal Analysis (OMA) is applied to the spectral characterization of the neutron noise. The OMA 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-3 K. The simulations considered different single-source fluctuations and their combinations. 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, which allows distinguishing the effects of different phenomena on the spectral characteristics at the same frequency range of the neutron noise. 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.