高温气冷堆石墨散体结构中窄缝旁流的建模与分析

The reactor core of the high temperature gas-cooled reactor (HTGR) is the loose structure of many graphite blocks. Among the graphite blocks are kinds of narrow gaps where the dense helium flow over and become the bypass flows in the reactor core. The existence of the bypass flows in gaps indeed affects the flow and temperature distributions in the reactor core, which is crucial to the reactor safety. Large amount of gaps are in small sizes and interconnected with complicated shapes, so that the flow types in the gaps as well as the flow distributions are not well understood. In our project, molecular dynamic method was adopted to investigate the wall effect of dense gas in order to summarize the flow boundary model for the computational fluid dynamic (CFD) simulations. With proper selection of gaps or gap networks, the inlet and outlet boundary conditions of their CFD models were employed to study the resistance performances of the dense gas flows in complex gaps with various sizes, in order to get the equivalent flow network models. With the combination of all the flow networks of gap bypass flows and the flow network of main flow through the pebble bed, the flow network of the reactor core was completed and detailed. Coupled with neutron kinetics and solid heat transfer calculations, the gap flow distributions were predicted in accuracy and the effect on multiple physics could be analyzed. Based on the complete flow network of the reactor core, the distributions and contributions of gap bypass flows to the total flow were analyzed to further developing the design and analyses of the HTGR.

高温气冷堆的堆芯是由大量石墨块堆叠而成的散体结构，稠密氦气流经石墨块间的窄缝，形成堆芯旁流，会改变堆芯的流量与温度分布，影响反应堆运行安全。窄缝的尺度小、数量多、结构与连通关系复杂，窄缝内的流动特性以及窄缝旁流的分布规律至今还不明确，需要深入研究。本项目采用分子动力学方法研究稠密气体的壁面效应，提炼的壁面模型可用于窄缝流的计算流体力学(CFD)建模计算；通过合理划分窄缝网络明确其CFD模型的进出口边界条件，进而研究不同尺度、复杂结构窄缝流的流动特性，并合理等效为流体网络模型；将所有窄缝的旁流流体网络与球床等主流通道的流体网络合并，建立完整而详尽的堆芯流体网络模型，并耦合中子动力学、固体传热等模型，可以实现窄缝旁流分布的准确预测及其对堆芯多物理场的影响分析，明确窄缝旁流的分布规律以及关键的窄缝旁流位置，从而促进高温气冷堆的深化设计与研究。

高温气冷堆的堆芯是由大量石墨块堆叠而成的散体结构，稠密氦气流经石墨块间的窄缝，形成堆芯旁流，会改变堆芯的流量与温度分布，影响反应堆运行安全。窄缝的尺度小、数量多、结构与连通关系复杂，窄缝内的流动特性以及窄缝旁流的分布规律至今还不明确，需要深入研究。本项目采用分子动力学方法建立了稠密氦气在石墨壁面间导热的分子模型，研究了气体分子在石墨壁面的能量交换情况并计算了热协调系数，其温度跳跃等壁面效应影响范围有限，在反应堆中毫米量级窄缝旁流通道中无影响；研究了稠密氦气在毫米量级石墨窄缝中的流动问题，通过深入的计算流体力学(CFD)建模分析并与实验结果对比，明确了边界层网格划分原则以及相关湍流模型的适用性，RNG k-ε模型结合增强壁面处理以及SST k-ω模型均能将压降计算误差控制在10%以内；研究了侧反射层竖缝旁流问题，分别建立其流体网络模型以及CFD模型，明确了堆芯不同高度球床与竖缝间的横向流动方向与流量大小，堆芯上部由窄缝流向球床，下部由球床流向堆芯，堆芯上部的结构对堆芯主要部分的旁流影响很小，旁流比例主要受竖缝尺寸大小影响，当竖缝尺寸在堆芯高度一致为1.6mm时，堆芯底部的旁流比例约为2%，随着竖缝尺寸的增大，旁流比例增高；针对高温气冷堆的堆芯结构进行了详细的流道分析，明确了主要的窄缝旁流通道及其连通关系，对局部窄缝开展CFD建模并获取其阻力特性，通过合理划分与等效，形成了节点数为6000的完整旁流流体网络模型，进一步与球床等主流通道的流体网络合并，并耦合中子动力学、固体传热等模型，实现了对不同反应堆运行状态下窄缝旁流分布的预测及其对堆芯多物理场的影响分析，总旁流比例在反应堆运行初期时约为5.74%，运行末期时为8.89%。本项目从基础研究出发，不仅揭示了复杂反应堆堆芯结构中的旁流分布规律，丰富了研究手段，加深了对高温气冷堆运行时热工流体状态的理解，也为推动高温气冷堆的工程实现、安全审评提供了有力支撑。