Research On Thermal-hydraulic Characteristics And Break Size Calculation Model Of Small Break Blowdown Process Of Pressurizer Water Side

The pressurizer is an important device for controlling the temperature and pressure of the primary circuit system in the nuclear power plant.When a small break accident occurs on the water side or steam side of the pressurizer,the temperature and pressure of the pressurizer and the primary circuit system will drop.If the accident is not dealt with in time,it may cause serious consequences.In this paper,the small break blowdown process of pressurizer water side is selected to study,trying to understand the variations of thermal hydraulic parameters during the small break blowdown process of pressurizer water side.Break parameters such as break size can be quickly calculated in the early stage,providing support for operators to take effective measures to reduce the impact of the accident in the early stage of the accident.This paper uses the methods of physical experiment,numerical experiment and numerical simulation to study the variations of thermal hydraulic parameters during the blowdown process of pressurizer water side.An experimental bench for the small break blowdown process of the pressurizer water side was designed and built,a numerical experimental Relap5 program for the small break blowdown process of the pressurizer water side and a numerical simulation model near the break position in the initial stage of the small break blowdown process of the pressurizer water side.were established at the same time.The method of physical experiment was used in the case of low pressure,and the method of numerical experiment was used in the case of high pressure to study the variation of the thermal hydraulic parameter characteristics during the small break blowdown process of the pressurizer water side,the relationship between thermal hydraulic characteristics such as container temperature,pressure,liquid level,pipeline mass flow and different parameters such as time,break diameter,initial pressure were studied respectively.The reasons for the variations were analyzed,and the laws of the relevant parameters were obtained,and the main sensitivity parameters affecting thermal-hydraulic behavior during blowdown process are identified as the rate of fluid temperature,the rate of pressure in the vessel and the rate of liquid level.In the numerical simulation study of variations of the thermal hydraulic parameter characteristics near the break in the initial stage of the small break blowdown process of the pressurizer water side,the influence of the nozzle diameters and blowdown pressures on the characteristics of the flow field average temperature,the average velocity of droplets and the average velocity of steam are obtained,and the main sensitive parameters affecting the thermal-hydraulic behavior near the break nozzle in the initial stage of the blowdown process are identified as average temperature of the flow field,the average velocity of the liquid phase and the average velocity of the vapor..On the basis of the experimental research on the variation of thermal hydraulic parameter characteristics during the small break blowdown process on the water side of the pressurizer,the paper establishes a mathematical and physical model of the small break blowdown process on the water side of the pressurizer through theoretical analysis,and a program was developed based on the model,which realized the fast calculation of important thermal-hydraulic parameters in the blowdown process of small break on the water side of the pressurizer.The study uses the physical experiment results under low pressure conditions and the Relap5 numerical experiment results under high pressure conditions to verify the rationality and accuracy of the mathematical and physical model of the small break blowdown process of the pressurizer water side proposed in this study,and the verification results show good rationality and accuracy.The paper applies the NSGA-Ⅱ genetic algorithm in the multi-objective optimization method,based on the mathematical and physical model of the small break blowdown process of the pressurizer water side,a break size calculation model for the small break blowdown process of the pressurizer water side is established combined with the NSGA-Ⅱ genetic algorithm.This calculation model takes the break diameter and gas mass quality as decision variables,and uses a multi-objective optimization method to predict the range of the break sizes.Comparing the calculation results of the model under low pressure conditions with the physical experimental results,and the calculation results under high pressure conditions with the numerical experimental results of Relap5,it is found that the calculation model can be used to calculate the break size quickly and accurately during the small break blowdown process on the water side of the pressurizer.