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

A finite-element model describing the mechanical vibrations of the whole WWER-440 pri- mary circuit was established to support the early detection of mechanical component faults. A specialfluid±structure module was developed to consider the reaction forces of thefluid in the downcomer upon the moving core barrel and the reactor pressure vessel. Thisfluid±structure interaction (FSI) module is based on an approximated analytical 2D-solution of the coupled system of 3Dfluid equations and the structural equations of motions. By means of the vibration model all eigenfrequencies up to 30 Hz and the corresponding mode shapes were calculated. It is shown that the FSI strongly in¯uences 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 ele- ments was investigated. Comparing the frequency spectra of the normal and the faulty struc- ture, 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.