Analysis Of AP1000 Loss Of Coolant Accident And Model Sensitivity Study

Small break LOCA is one of the PWR design basis accidents,experimental researches and simulations had proved sufficient capacity of AP1000 passive safety system to mitigate consequence of accident in previous.During the period of AP1000 passive core cooling system participate in small break LOCA,each key equipment in reactor primary coolant system occur complex transient,such as the critical flow at the break,the water level swelling in the core,the vapor-liquid countercurrent in the surge pipe,and the droplet entrainment in the hot leg,etc.The research and analysis of these complex thermal hydraulic behaviors will help us understand the complexity of reactor operation and the stability of passive safety system better.In this research,RELAP5 code is used to model the primary coolant system and passive core cooling system of AP1000,and a reasonable steady-state model is obtained.Taking the cold leg 10 inches break as an example,the emergency response and mitigation ability of the passive core cooling system are verified.Sensitivity studies were done at pressurizer surge pipe and hot leg connecting ADS stage 4 valve.The former has the phenomenon of vapor-liquid countercurrent during the pressurizer re-drain,while the latter has the phenomenon of droplet entrainment after ADS stage 4 is opened.In this research,sensitivity of node and structural model of the surge pipe are studied.With the decrease of the control volume size of the surge pipe,the duration of the pressurizer drain and the primary loop pressure relief are delayed to a certain extent,and so is IRWST injection.When L/D of the control volume is set about 1.1,the influence of subsequent physical processes is no longer obvious.According to the actual structure of the surge pipe,the complexity of the simulation model of the surge pipe is modified.The addition of elbows will affect the call of RELAP5 code to CCFL subroutine,delay the emptying time of pressurizer,and finally affect the start-up time point of IRWST injection,the difference is about 1.1%.In the comparison results of whether the droplet entrainment model is added at the T-junction of ADS stage 4 valve,adding the droplet entrainment model can increase the pressure relief rate of 40% of the coolant system,gravity injection of the IRWST earlier,and the injection flow curve is more stable.