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模拟沸水堆中支撑核燃料棒的隔离器的圆柱形流障。通过高速摄像机仔细观察了流动模式，特别是干斑和燃尽的产生。同时测量了在间隔片附近6个轴向不同位置上加热管表面温度的时间变化。结果总结如下：(1)较大幅度的温度变化是由于在干斑形成之前，通过相当薄的液膜的换热迅速增加，而干斑由于换热恶化，导致温度迅速升高。(2)在加热试验段入口过冷的情况下，当表面蒸汽速度jG在15 ~ 20m/s左右时，干斑的出现造成的温差最大，最多为5℃。在jG >条件下，由于环形流型更加充分发展和稳定，热流密度增加，但热流密度减小。干斑的产生是由前一种情况下的排水和基膜液的蒸发引起的。(3)当孔隙比为正时，入口流型为段塞流，流体流速较低（jL = 0.1 m/s），整体流动结构变得不稳定，发生干斑时最高温度达到140℃。提出了dtypatch生成的区域图。(4)在热流增加和减少两种情况下，温度都逐渐接近最终状态。(5)进口质量温度在突然升高的情况下也逐渐发生变化，而在突然降低的情况下受热面温度则急剧升高。温度降低越大，越容易发生烧断。

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.