Due to the inherent safety characteristics of passive core injecting system, it is one of the most important mitigation strategies under loss of coolant accident (LOCA). However, flow instability during passive gravity-driven reflooding may occur, which can obstruct core reflooding process and reduce the heat removal rate from the reactor core. Therefore, exploring the characteristic and influencing mechanisms of flow instability during gravity-driven reflooding is necessary to analyze the effectiveness of passive core injecting system and mitigate the consequences of LOCA.

In this work, small scale experiments are performed to investigate the flow instability during gravity-driven reflooding when the open ratio of steam outlet are in the ranges of 3.33–50%, gravity driving head are 16–20kPa and system pressure are 0.1–0.3MPa. The test facility consists of a cooling water storage tank, a test section and a condensate tank. The results show that flow instability could occur under specific conditions and have a strong regularity, which is divided into three stages: cooling water initial injection, cooling water expulsion and cooling water re-injection. In addition, the effects of open ratio of steam outlet, gravity driving head and system pressure on flow instability phenomenon are investigated. Increasing the open ratio of steam outlet accelerates the discharge rate of steam and prevents the accumulation of steam at the inner cylinder, which can contribute to cooling the heated rods and restrict the flow instability. And under low gravity driving head of 16kPa and low system pressure of 0.1MPa the temperature and pressure acutely change and flow instability is more likely to occur.

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