Cliff-edge Effect When Operating NPP with VVER

In this paper, the main reasons for the excitation of vibrations in the main circulation circuit (MCC) equipment of VVER-1200 NPP were investigated. Using an interdisciplinary methodology, digital acoustic models (DAMs) of the MCC and pressurizer system (PS) were created and proved to be self-oscillating systems that generate acoustic standing waves (ASWs) similar to simultaneous operation of several Helmholtz resonators, each of them is capable of suppressing ASWs appearing within MCC. ASW frequencies' calculation is done by Thomson formula. Analysis of hazardous vibrations' reasons in the equipment of VVER-1200 during earthquakes was carried out. It was discovered that the main cause is cliff-edge effect (СЕ) which takes place at the event of surface boiling inside reactor core (RC), which was not considered as regulation documents were prepared. It was concluded that resonances of frequencies that coincide with vibrations of the MCC equipment and structure added to the newly identified СЕ, decrease capacity factor (CF) of plants with VVER-1200 compared to plants with VVER-1000.