Level swell analysis of stagnant water pool in filtered containment venting systems

摘要

A filtered containment venting system (FCVS) prevents over pressurization of containment vessel and releasing of radioactive materials during the severe accidents in nuclear power plants. During the venting process, it has been reported that the two-phase mixture level in a wet FCVS tends to swell and fluctuate. The behaviors depend on inlet/boundary conditions and physical properties of the injected gas, which vary as the accident progresses. Proper controlling and monitoring of the FCVS pool water level is crucial because it affects filtration performance including scrubbing process and thermal-hydraulic stability. In order to investigate this phenomenon, the current study proposes a set of nitrogen and steam injection experiments using a vertical pipe with a diameter of 105 mm to evaluate the effects of flow conditions and physical properties of gases. Drift flux analysis was carried out to predict the two-phase mixture water level and its fluctuations. The experimental two-phase mixture level was consistent with the values predicted by the drift flux model for nitrogen and steam injection, and the model's capability was confirmed for the system pressure ranging from atmospheric to 0.20 MPa and initial water level ranging from 0.6 to 2.6 m for both small and large diameter pipe configurations. The fluctuation amplitudes in the current experiment were smaller than those observed in experiments conducted on small-diameter pipes. The mean two-phase mixture water level increased upon pressurization of the scrubbing pool. However, it was found that the effect of pressurization on the two-phase mixture level fluctuation amplitude was negligibly small.