INSTABILITY OF TWO PHASE FLOW IN A 3-D NATURAL CIRCULATION COOLED NUCLEAR REACTOR SYSTEM

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To assure the passive-safety of the SBWR cooled by natural circulation its possible instabilities are examined with using a small scale experimental apparatus. This is a 1+3 dimensional system, composed of a glass container with three heater parallel channels, and a chimney volume on the top half of the container.Experiments were performed for various number of channels to observe closely the instabilities : the geysering and the density wave oscillation. It was clarified that the geyserings are initiated either from the top or the bottom of the heated channels, with accelerated propagation of the boiling front due to the gravity-governed self-evaporation. On the other hand the feedback effect between the driving force of the natural circulation and the movement of the bubbles plays the dominant role. It was very often to observe these two type instabilities occur simultaneously. These observations were simulated with using a newly developed time-domain computer code.Experiments were also performed with using a larger container with higher chimney volume. Although with a higher chimney, it was observed that the natural circulation flow rate unchanged. This was because, the secondary circulation in the chimney volume makes the pressure drop uniform, which declines the driving force of the natural circulation in the chimney part. This indicates that dividing of the chimney with several vertical segments is advisable. This observation was verified by analyses with a computer code using the drift-flux model.