Theoretical Vibration Model of VVER Reactors Considering Fluid-Structure-Interaction

A finite-element-model describing the mechanical vibrations of the whole VVER-440 primary circuit is being developed to support the early detection of mechanical component faults. A special fluid-structure-element was developed to consider the reaction forces of the fluid in the downcomer upon the moving core barrel and the rector pressure vessel. It was derived from 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 eigenfrequencies up to 30 Hz and the corresponding mode shapes were calculated. It is shown that the fluid-structure-interaction strongly influences those modes exhibiting 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. By comparing the frequency spectra of the normal and of the faulty structure, it could be shown that a recognition of such degradations and failures even inside the reactor pressure vessel is possible by pure excore vibration measurements.