A L~NEAR DYNAiviiC AiiAiYSiS OF VENT CONDEi\iSATiOi\i STABiiiTY

Pressure suppression systems in boiling water reactors a r e designed to condense a largearmunt of steam very rapidly by injecting it into a pool of water. It transpires that such condensing flows a r e unstable and can lead to large oscillatory pressures on the walls of the containment system. This paper presents a theoretical model whose purpose i s to attempt to understand why these flows are unstable and to extract the important parameters and frequencies pertaining to the instability. A simple linear dynamic model i s constructed comprising linear transfer functions for (i) the unsteady steam flow in the vent (ii) the condensation interface and (iii) the pool hydrodynamics. The analysis demonstrates the existence of both stable and unstable regions of operation defined by several non-dimensional parameters including the ratio of the steam flow rate to the effective thermal diffusivity in the water just downstream of the condensation interface and the frictional 10s ses in the vent. Instability frequencies are in the vicinity of the vent acoustic frequencies or the pool manometer frequency depending on the conditions. Though the qualitative dynamic behavior of the model i s consistent with the experimental observations, quantitative comparison is hindered by difficulties in accurately assessing the effective thermal diffusivity in the water. Nevertheless the model provides insight into the nature of the instability.