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

为了确保自然循环冷却SBWR的被动安全性，利用小型实验装置对其可能的不稳定性进行了考察。这是一个1+3维系统，由一个带有三个加热器平行通道的玻璃容器和容器上半部的烟囱体积组成，对不同数量的通道进行了实验，以密切观察喷泉和密度波振荡的不稳定性。结果表明，间歇泉是从加热通道的顶部或底部开始的，由于重力控制的自蒸发，沸腾锋面的加速传播。另一方面，自然循环的驱动力与气泡运动之间的反馈作用占主导地位。经常观察到这两种类型的不稳定同时发生。使用新开发的时间域计算机程序模拟了这些观测结果，并使用更大的烟囱体积更大的容器进行了实验。虽然烟囱较高，但观察到自然循环流量不变。这是因为，烟囱体积内的二次循环使压降均匀，从而降低了烟囱部分自然循环的驱动力。这表明，将烟囱分成几个垂直段是可取的。这一观察结果通过使用漂移流模型的计算机代码的分析得到了验证。

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.