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

A finite-element-model describing the mechanical vibrations of the whole WWER-440 primary circuit was established to support the early detection of mechanical component faults. A special fluid-structure module was developed to consider the reaction forces of the fluid in the downcomer upon the moving core barrel and the rector pressure vessel. This fluid-structure interaction module is based on an approximated analytical 2D-solution of the coupled system of 3D fluid equations and the structural equations of motions. By means of the vibration model all igenfrequencies up to 30 Hz and the corresponding mode shapes were calculated. It is shown that the fluid-structure interaction strongly influences those modes that lead to 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, it could be proved that a recognition of such degradations and failures even inside the reactor pressure vessel is possible by pure excore vibration measurements.