First evidence of the pivotal motion ("tilting mode") of the core Barrel in the RINGHALS-4 PWR

The Division of Subatomic Physics and Plasma Physics (formerly Division of Nuclear Engineering) in Chalmers, Göteborg, and the Ringhals Nuclear Plant have investigated the core barrel vibrations in the Ringhals PWRs over the last 20 years. Based on the different symmetry properties of the vibration modes, a mode separation technique was developed to enhance the contributions from the different modes. Recent observations of wear at both the lower and upper core-barrel-support structures in the Ringhals PWRs indicated that vibration modes of the core barrel other than pendular (beam mode) and shell mode are likely to occur. A beam mode type movement alone is not able to explain such a wear, and therefore, it is fair to assume that the vibration mode in question is a small amplitude periodic tilting movement of the core barrel around a horizontal, diagonal pivot at the half height of the core. In this work, ex-core data taken in the Ringhals-4 PWR were analyzed in order to find evidence of such a tilting movement. First, cross spectra between various ex-core detectors were calculated and analyzed to locate the frequency range of the new vibrational mode. Then, a model based on symmetry considerations was derived in order to extract the sought mode from the spectra. The measurements were evaluated by the new mode enhancement technique. The results show that it is possible to enhance such a mode and find it in the spectra by properly combining the signals in the time domain