Fluid structure interaction in the pool of pressure suppression systems containing oscillating bubbles

During a loss-of-coolant-accident or after safety relief valve actuations the water in the pool of a BWR-pressure suppression system is containing gas bubbles with under or overpressure causing loads on the surrounding pool walls which have a certain dynamical response. On the other hand there is also a minor response of the bubbles on the wall motion. As a result the mutual influence between the bubble and the wall, which might be called 'bubble-fluid-structure-interaction' (BFSI), leads to a corresponding change of the eigenfrequencies of the whole mechanical system. In order to investigate the effects of BFSI an analytical approach has been developed based on principles of potential theory and Lagrangian dynamics. The bubble-fluid-structure continuum is mathematically described by a set of generalized coorddinates such that space and time dependent calculations are performed separately. The space dependence is obtained by calculating stationary velocity potentials under adequate boundary conditions. The time dependent problem is solved by integration of coupled ordinary differential equations. Results are presented for a single spherical locally fixed bubble interacting with isolated structural mode. If the bubble frequency is smaller than the frequency of the unperturbed wall vibration the wall frequency is increased by the presence of the bubble. Otherwise it is decreased. The bubble frequency itself decreases in the first and increases in the latter case. (orig.)