Research Of Flow Instability In Natural Circulation Loop With Parallel And Rod Bundles Channels

The natural circulation(NC)simplifies the nuclear reactor system and improves its inherent safety and passive quality.There are parallel pipes and rod bundles in the nuclear power plant equipment.Flow instability could occur in these channels during the operation.In this dissertation,the flow instability in NC loop with two sorts of channels,i.e.parallel pipes and 3×3 rod bundles are analyzed.The NC system with parallel pipes is studied by RELAP5 code,the rod bundles of NC system is studied by experiment and RELAP5 code.In the research of NC system with parallel pipes,firstly,the NC system with single pipe is modeled by RELAP5 code;then the RELAP5 results are compared to the experimental results in order to validate its accuracy and reliability;finally,based on the NC system with single pipe,the NC system with parallel pipes is modeled.The in-phase and out-phase flow instability oscillation are researched by the method of driving force,the pressure drop oscillation(PDO)occurs in each channel and its mechanism and instability boundary are obtained.The mass flow rate of parallel pipes has the trend to keep the same oscillation type under NC condition.The in-phase flow oscillation includes boiling,pulsation between pipes,flashing and condensation.The pulsation between pipes leads to the large flow oscillation.A period of out-phase oscillation is selected to analyze its flow oscillation characteristics.The instability boundary in NC with the parallel pipes is obtained.The research of the NC system with 3×3 rod bundles is divided into two parts:experiment and RELAP5 simulation research.When the pressurizer is located in the downstream position of the heated section,the flow instability phenomenon is studied.Analyzing the relationship between the pressure drop of the heated section and the mass flow rate,a slender close curve is found.In the theoretical analysis of PDO,this kind of relationship should be a parallelogram,but their mechanism is almost the same.By further analyses,this flow instability oscillation is proved to be the PDO.Based on the experimental data,the NC system with rod bundles is modeled by RELAP5code.By comparing the experimental results and the RELAP5 simulative results,the model is proved to be accurate and reliable.This model is used to further analyze the whole system under higher heat flux conditions(the maximum experimental heated flux of the rod bundles is 221.05 kW/m~2)and some parameters that are difficult to obtain under the experimental condition,such as the density and the water level of the pressurizer.The instability boundary of the NC system with 3×3 rod bundle channels is obtained.The applicability of RELAP5under low pressure NC condition has been validated.