Time-frequency analysis of non-stationary neutron noise in a small modular nuclear reactor

Abstract In this work, for the first time, the time-frequency analysis technique is proposed to analyze a non-stationary neutron noise. The work investigates on an appropriate way to solve the neutron noise equation in the time-frequency domain in an SMR reactor. To actualize this procedure, a computer program has been developed. This program is based on the use of three main modules. The first is a static calculation by a finite difference method and a power iteration algorithm for providing a database of the required neutronic parameters. The second is the neutron noise calculations in the time-frequency domain and the extraction of the noise characteristics by using the Wigner-Ville distribution. The third is the categorization of the neutron noise sources based on their extracted characteristics such as the source type, the source location, the number of sources, the time of the occurrence, the instance frequency and the noise amplitude. In the first module, the static calculations are benchmarked by the referenced values. Then, in order to validate the static noise calculation, the obtained result is compared to the result of the noise calculation in zero frequency. The results of adjoint and direct methods are compared with each other to confirm the validity of the dynamic noise calculation. The obtained results show a good agreement with each other. In addition, the obtained results of the proposed method and Fourier method are compared with each other at the initial time. Moreover, various noise sources, including mono and multi-frequency, are solved in the time-frequency domain to evaluate the proposed method’s capability. All results show that the proposed method has a good capability to solve neutron noise equation. The results obtained show that the time-frequency method gives comprehensive results in comparison with frequency methods. Finally, it is concluded that analyzing the noise sources in the time-frequency domain can be used as a remarkable approach to investigate the core behavior and to recognize the non-stationary processes in the core.

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