Abstract
KWU-PWR reactors (SIEMENS design) are commonly exhibiting high neutron noise levels that can lead to costly operational issues. Recent analysis seems to indicate that, coolant flow, temperature oscillations, and mechanical vibrations have a key impact on neutron noise phenomena. In order to advance in understanding this phenomenon, the transient nodal code SIMULATE-3K (S3K) has been already utilized to simulate scenarios with individual or combined types of perturbation sources: mechanical vibrations of fuel assemblies and thermal-hydraulic fluctuations at the core inlet. In this work, new simulations are performed with all the perturbations applied simultaneously. The simulated neutron detectors responses are then analyzed with noise analysis techniques. All the simulated spectral features of neutron noise are compared to those obtained from real plant data. Results show that the simulated neutron noise phenomenology behaves similarly to that obtained from real plant data by increasing the fluctuation amplitude in the inlet coolant flow in the S3K calculations.
Highlights
The normalized root mean square (NRMS) of neutron detectors signals have experienced an increase in certain German, Spanish and Swiss pre-Konvoi and Konvoi pressurized water reactors (PWRs) [1,2,3,4]
The auto power spectral density (APSD), NRMS distribution, and the coherence and phase functions obtained for Scenario A are very similar to those of the scenario where only temperature fluctuations are assumed [5]
The following observations can be made on the APSDs and NRMS: - The thermal-hydraulic perturbations cause a strong low-frequency spectrum below 1 Hz, as observed in B.2 and C.2 scenarios. - The neutron noise variance (The area of the APSD) has a similar trend in all the scenarios B and C, and it is directly related to the distance of the in-core detectors from the sources of perturbations. - In scenarios C, we can see a faster decrease of the APSD as the frequency increases
Summary
The normalized root mean square (NRMS) of neutron detectors signals have experienced an increase in certain German, Spanish and Swiss pre-Konvoi and Konvoi pressurized water reactors (PWRs) [1,2,3,4]. The simulated data were analyzed by means of noise analysis techniques and several spectral features were found similar to those of the real plant data. The simulated scenarios are based on the combination of two and three types of perturbations, i.e. fuel bundle lateral vibrations, and inlet coolant flow and temperature fluctuations. The amplitude of these perturbations have been modified in every simulation to observe the differences in the spectral characteristics of neutron noise. Due to the high number of simulated scenarios, the observed neutron noise spectral features have been organized properly in order to perform a meaningful assessment
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