Finite element-based vibration analysis of WWER-440 reactors

A finite element model describing the mechanical vibrations of rite whole W WER-440 primary circuit was established to support the early detection of m echanical component faults. A special fluid-structure module was developed to consider the reaction forces of the fluid in the downcomer upon the movi ng core barrel and the reactor pressure vessel (RPV). This fluid-structure interaction (FSI) module is based on an approximated analytical two-dimensi onal solution of the coupled system of three-dimensional fluid equations an d the structural equations of motions. By means of the vibration model all eigenfrequencies up to 30 Hz and the corresponding mode shapes were calcula ted It is shown that the FSI strongly influences those modes that lead to a relative displacement between the RPV and the core barrel. Moreover, by me ans of the model, the shift of eigenfrequencies due to the degradation or t o the failure of internal clamping and spring elements was investigated. Co mparing the frequency spectra of the normal and the faulty structure, one c ould prove that recognizing such degradations and failures even inside the RPV is possible by pure ex-core vibration measurements.