REFINED METHOD FOR SURVEILLANCE AND DIAGNOSTICS OF THE CORE BARREL VIBRATIONS OF THE RINGHALS PWRs

Surveillance and diagnostics of core barrel vibrations has been performed in the Swedish Ringhals PWRs for several years, with main focus on the pendular motion (beam mode). The monitoring of the beam mode showed that the amplitude of the corresponding peak in the ex-core neutron spectra increases along the cycle, and decreases after refueling. Previous investigations on the reason of this behaviour, i.e. whether it is due to the increase of the core barrel vibration amplitude or to the increase of the neutron physics coupling between vibrations and neutron noise, were not decisive. The objective of the work reported here is to clarify this question. From frequency analysis, two modes of vibration have been identi-fied in the frequency range of the beam mode. Several results coming from the trend analysis performed during recent years indicate that one of the modes is due to the core barrel motion itself and remains constant during cycle, and the other is due to the indi-vidual flow induced vibrations of the fuel elements, showing an increasing trend during the cycle. In this work, the method to separate the contributions from the two modes has been refined, and the results of this approach to the latest measurements are presented. The results confirm the origin of the two vibration modes and show constant amplitude of the core barrel motion throughout the cycle