Pressure distribution due to a steam bubble collapse in a BWR pressure suppression pool

Abstract

At the end of a steam blowdown into a pressure suppression pool severe steam bubble collapses occur. Because the condenser pipes are arranged in a regular pattern. There are two limit loading conditions: the simultaneous collapse with the same intensity at each condenser pipe which is mostly discussed and a strong collapse only at one pipe or a small group of pipes. Since the influence of the wall elasticity on the pressure input due to the steam bubble collapse is unessentially small, the complete bubble-fluid-structure problem may be solved in two steps: First the bubble-fluid interaction in the rigid containment and later on the fluid-structure interaction in the elastic containment due to the pressure input by the bubble-fluid system. The bubble-fluid problem is also separated in the analysis for the pressure time history and that for the pressure distribution. If compressibility effects are neglected, the steam bubble surface is to idealize as a sphere during the whole collapse time for analyzing the pressure time history. The pressure distribution is fixed then by the potential due to a stationary source in the bubble center. The structure system is solved as usual in structural dynamics by the eigenmode response analysis in which the fluid-structure system is substituted by some uncoupled mass spring oscillators. The aim of this paper is to clarify the behaviour of the bubble-fluid-structure system by simple models and by separating the problem into the important global behaviour and some additional localized effects. This seems to be necessary since the pool geometry especially for steel containments and also the analysis for the structural system, which consists of different shells is too complicated. The influence of different idealisations on the pressure distribution in the rigid containment and on the eigenmodes of the fluid-structure system is presented. Formulas are derived for estimating the eigenfrequencies, the eigenmodes and the pressure distribution as well as the pressure reduction by the wall elasticity. Furthermore the similarity rules for transfering the results from small experimental facilities to the complex ring containment are explained. Analytical and experimental results are compared and how the maximum pressure distribution in steel and concrete containments differ from each other. Finally the following engineering concept for a stepwise analysis is confirmed: The resulting wall loads due to the fluid-pressure and the inertia wall forces may be determined by a simple fluid-structure model and then the stresses due to these loads by a refined shell analysis.