Theoretical vibration model of WWER reactors considering fluid-structure interaction

A finite-element model describing the mechanical vibrations of the whole WWER-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. 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. 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 ex-core vibration measurements. (author)