Finite element-based vibration analyses of WWER-440 type reactors

To support the early detection of mechanical component faults at WWER-440 reactors a finite-element-model describing the mechanical vibrations of the whole primary circuit was established. A special element was developed to describe the fluid-structure interaction in the downcomer. It is based on an approximated analytical 2D-solution of the coupled system of 3D fluid equations and the structural equations of motions.
The vibration modes up to 30 Hz were calculated. It is shown that the fluid-structure interaction strongly influences those modes with a relative displacement between reactor pressure vessel and core barrel.
Moreover, by means of the model the shift of eigenfrequencies due to the degradation or to the failure of internal clamping and spring elements was investigated. Comparing the frequency spectra of the normal and the faulty structure, one could prove that a recognition of such degradations and failures even inside the reactor pressure vessel is possible by pure excore vibration measurements. The results hint at the
opportunity to establish a monitoring procedure that is capable of distinguishing between guide lug and spring pipe failures, because these two types of failures are mapped in different and well separated frequency regions. Moreover, it seems that it is possible to detect these failures by pure excore vibration measurements, what is important since incore vibration measurements are difficultly to be implemented over longer time periods. When measuring the displacements for x and y-direction even the localization of the failed guide lug might become achievable.