Research On Control Strategy After Condensate Pump Trip In AP1000

The condensate pump in nuclear power plant is prone to trip because it works in a high-pressure environment for a long time.This failure will cause the water level of the deaerator to decrease,which will trigger the trip of the feed pump and affect the SG water level.the current control strategy of the nuclear power plant is to reduce the load to reduce the main supply water demand of the nuclear island and stabilize the water level of the deaerator and SG.However,if the condensate pump is not properly handled after tripping,there is still a risk of reactor shutdown,which will cause economic losses to the nuclear power plant.Currently,most studies on the corresponding control strategy are based on conservative assumptions to determine the load reduction rate,but the change process of key parameters during the load reduction process has not been quantitatively evaluated.In addition,the impact of the corresponding protective system response on the nuclear power plant after the condensate pump trips has not been considered.This process involves multiple protective signals in the primary and secondary loops,such as steam generator water level,deaerator water level,condenser water level,over-temperature protection signal △T,coolant reactivity feedback,etc.This work will simulate the change process of key parameters after the condensate pump trips based on a coupled calculation method of the primary and secondary loops,and determine the change process of key parameters under different load reduction rates,providing some guidance and data support for the control strategy after the condensate pump trips in nuclear power plants.This paper takes AP1000 as an example,based on FLOWNEX and RELAP 5/SC DAP SIM mod3.4 software for AP1000 one-two circuit system modeling.According to the tripping condition of the condensate pump,Transient calculations were carried out for the condensate pump trip scenario,with control system inaction,simple action,and different load reduction rates(3%,5%,10%,20%),obtaining the change of key parameters in the primary and secondary loops.The results show that:(1)If the control system is inactive,under the action of the deaerator water level protection system,the outlet valve of the condensate pump will automatically open,causing the last condensate pump to trip due to excessive flow,and the reactor will shut down due to low SG water level in 185.8 seconds after the accident.(2)If the control system takes simple action to ensure the normal operation of the last condensate pump,the deaerator water level will drop due to the loss of half of the condensate water,triggering the feedwater pump trip and affecting the SG water level,leading to reactor shutdown due to low SG water level at 313 seconds.(3)When performing load reduction operations,load reduction rates below 5%/min will cause the reactor to shut down due to low SG water level.Load reduction rates greater than or equal to 5%/min can enable the reactor to smoothly enter low power operation,but the faster the load is reduced,the greater the coolant temperature reactivity feedback introduced.