喷淋冷却条件下稠密乏燃料棒束间孤波粘连流动特性及其对传热性能的影响研究

Spray cooling technology has been applied to the spent fuel safety design of AP1000 designed by Westinghouse Electric Company and CAP1400 China's self-developed and third-generation reactors. Spent fuel is cooled by spray water in the extreme accident with loss of coolant in spent fuel pool. When the spray flow reaches a certain value, solitary wave adhesion will occur between adjacent rods. This excessively severe disturbance will cause temporary or permanent breakdown of partial liquid film, which will lead to heat transfer deterioration and threaten the safety of the spent fuel. At present, the research on the solitary waves of liquid film mainly focused on the individual geometry of a single rod or a plate. However, there is a lack of studies on the effects of adhesion between solitary waves in dense rod bundles on flow and heat transfer of liquid film. Based on the safety design of spent fuel spray cooling as the research background in AP1000 nuclear power plant, an experiment was performed on the existing spent fuel spray cooling test bench. A combination of high-speed cameras and remote microscopes was selected as a measurement method. Firstly, the formation and evolution of solitary wave adhesion between dense spent fuel rod bundles are discovered. Second, the droplet entrainment characteristic caused by impact on spacer from solitary wave adhesions also is obtained. At last, by clarifying the effect of solitary wave adhesion on liquid film heat transfer, a critical heat flux relationship is established for permanent breakdown of liquid film considering the effect of solitary wave adhesion between dense spent fuel bundles.

喷淋冷却技术已被应用于引进西屋公司的第三代AP1000和我国自主研发的CAP1400堆型的乏燃料安全设计中。在乏池失水的极限事故下，通过喷淋水来冷却乏燃料，当喷淋流量达到一定值后，相邻棒间将会产生孤立波粘连现象，这种过分剧烈的扰动会导致液膜局部暂时甚至永久破断，进而引发传热恶化，影响乏燃料的安全性。目前，针对液膜孤立波的研究主要集中于单棒或平板的单独几何结构，缺少稠密棒束间孤立波发生粘连后对液膜流动特性和换热特性影响的研究。本课题以AP1000压水堆核电站乏燃料喷淋冷却的安全设计为研究背景，利用现有的乏燃料喷淋冷却试验台架，基于高速摄像机和远程显微镜相结合的测量技术，揭示稠密乏燃料棒束间液膜孤波粘连的形成及演化规律，获得孤波粘连体冲击定位格架引发的液滴夹带特性，并阐明孤波粘连效应对液膜换热的影响，最终建立稠密乏燃料棒束间考虑孤波粘连效应的液膜永久破断临界热流密度关系式。

喷淋冷却技术已被应用于引进西屋公司的第三代AP1000和我国自主研发的CAP1400堆型的乏燃料安全设计中。在乏池失水的极限事故下，通过喷淋水来冷却乏燃料，当喷淋流量达到一定值后，相邻棒间将会产生孤立波粘连现象，这种过分剧烈的扰动会导致液膜变薄甚至破断，进而引发传热恶化，影响乏燃料的安全性。本项目首先基于单棒实验研究，获得液膜流动时空分布规律，得出当Re在608~7 538的范围内时，瞬态液膜厚度最大值出现在Re=7085的条件下，其值为2.36mm；随着Re的增加，时均液膜厚度会随之增加，并且液膜波动的振幅也会随之增加；在沿棒方向上，随着距棒顶距离的增加，液膜厚度会逐渐减小并趋于平稳，并且随着Re的增加，平稳部分会出现在距棒顶更远的位置；其次，基于2×2稠密乏燃料棒束试验，获得孤波粘连形成，演变及耦合机理，得出在发生孤波粘连现象时，周围液膜会被拉薄甚至破断，拉薄处的液膜为区域内最大液膜厚度的34.6%~44.9%。乏燃料棒上发生孤波粘连现象，会将粘连处周围的液膜拉薄超过一半的程度，对喷淋冷却时液膜的完整性存在一定的威胁。在某一条件下，液膜耦合现象出现之后并不会一直持续发展，而是会在一个极短的时间内快速充分发展。并且耦合区域会随着发展而逐渐扩大，且随着液膜的流动而进行移动。耦合区域外的液膜并不会受到耦合现象的影响而产生较大的变化。但在该条件下，随着液膜的不断流动，液膜偶合现象会多次产生，并且每次产生的耦合区域的范围大小会有所差异，耦合的发展时间也会存在不同；最后，基于5×5稠密乏燃料棒束试验段，通过多因素敏感性试验，获得包壳限值温度与喷淋流量之间的关系，从而建立稠密乏燃料棒束液膜换热关系式。项目结果可直接应用于大型压水堆乏燃料喷淋冷却系统的设计，在保证乏燃料系统安全的条件下，延长乏燃料喷淋冷却时间，保证事故后72小时乏燃料系统不需要运行人员操作的安全性。