RESEARCH ON THE GENERATION MECHANISM OF BURNOUT JUST UPSTREAM OF THE SPACER SUPPORTING BWR-NUCLEAR ROD BUNDLE

在垂直环形通道中安装了一个圆柱形流动障碍物，模拟了沸水堆中支撑核燃料棒的间隔件。用高速摄像机对流场进行了细致的观察，特别是对干斑和燃尽的产生进行了研究。同时测量了加热管表面温度随时间的变化规律。结果总结如下。(1)大幅度的温度变化是由于在干斑产生之前通过相当薄的液膜的热传递的快速增加引起的，这导致由于热传递恶化而导致的温度快速增加。(2)在加热段入口过冷的情况下，当表观汽速jG为15 ~ 20 m/s时，干斑产生的温差最大，最大可达5℃。在jG > 20 m/s的条件下，虽然热流密度增加，但由于环形流型的充分发展和稳定，热流密度降低。干斑的产生是由前一种情况下的排水和基膜液体的蒸发引起的。(3)在空隙率为正值、入口为弹状流、液体流速较低（jL = 0.1m/s）的情况下，整个流动结构变得不稳定，干堵发生时的最高温度可达140℃。提出了dtypatch生成的区域图。(4)在热通量突然变化的情况下，无论是增加还是减少，温度都逐渐接近最终状态。(5)在进口质量温度突然升高的情况下也逐渐变化，而在其突然降低的情况下受热面温度又突然大幅度升高。温度下降越大，越容易发生烧坏。

A cylindrical flow obstruction which simulates a spacer supporting nuclear fuel rods in BWR was installed in the vertical annular channel. Flow patters, especially the generation of drypatch and burnout, were carefully observed by a high speed video camera. The time variations of temperature on the heating tube surface were simultaneously measured near the spacer at six axially different locations. The results are summarized as follows.(1) Large amplitude temperature change is caused by rapid increase in heat transfer through quite thin liquid film prior to the generation of drypatch which causes the rapid temperature increase due to deterioration of heat transfer.(2) In the case that the liquid was sub cooled at the inlet of the heating test section the temperature difference caused by the occurrence of drypatch was the largest, at most 5℃, when the superficial vapor velocity jG is around 15〜20m/s. Under the condition of jG > 20 m/sit decreased even though the heat flux increased because the annular flow pattern became more fully developed and steady. The generation of drypatch is caused by the drainage in the former case and vaporization of base film liquid.(3) In the case that the void ratio was positive, the flow pattern was slug flow at the inlet and the liquid flow rate was low (jL = 0.1 m/s), the overall flow structure becomes unstable and the maximum temperature attains 140℃ when the drypatch occurred. The area map of the dtypatch generation was proposed.(4) The temperature gradually approached to the final state in the case of sudden change in the heat flux in both cases of increasing and decreasing.(5) In the case of sudden increase in inlet quality temperature gradually changed also, while in its sudden decrease the large and sudden increase in the heating surface temperature. The larger the temperature decrease the easier the occurrence of the burnout.