AP1000 Passive Containment Cooling System Engineered Safety Feature Actuation And Post-Accident Monitoring Reliability Research

As a representative of the world-wide third generation nuclear power plant model,AP1000 makes full consideration of the conception of passive in the design of the engineered safety feature(ESF),and makes use of gravity,natural circulation etc.methods to mitigate the consequence of the design basis accident(DBA)without human intervention.Passive Containment Cooling System(PCS)is a typical ESF system in AP1000,when the containment pressure reaches the ESF actuation setpoint under DBA,open engineered safety isolation valves,and the cooling water spay to the surface of the containment to prevent the in-containment pressure and temperature too much high.During a period of time after DBA,part of instruments is used for long term post-accident monitoring system(PAMS),and provides the data information for the emergency management after accident.It can be seen that whether ESF actuation or PAMS,their reliabilities play a vital role for the nuclear power plant safety during and post accident.This paper,from the point of seven containment pressure monitoring instrument,take the PAMS and ESFAS function of which perform as the object of study,using different reliability modeling method,make the research and analysis to their reliability.From the point of the PAMS reliability research,analyzes the reliability of PAMS instrument and PAMS channels separately.From instrument perspective,introduces the PAMS variables classification in the design of AP1000,instrument related test requirement and evaluate the impact on the PAMS instrument accuracy to the safety function actuation.From monitoring channel perspective,by using fault tree(FT)model establishment,modeling and analyze various signal transmission methods for each division.Compare the merits of the reliability of each division via analysis and quantitative calculation.As a whole,the reliability of AP1000 containment pressure PAMS reaches a very high level.From the point of ESFAS reliability research,this paper uses dynamic flowgraph methodology(DFM)which is recommended in NRC report.This method not only embodies the dynamic characteristics of the system better than the traditional FT method,but also reflects variation and association between the conditions of the nodes in both process system and instrument and control(I&C)system simultaneously.In the study of this paper,successfully establish the DFM model for ESFAS of the AP1000 PCS system,and definite the condition of each variable node and decision table.Finally,thru the conversion to timed fault tree,using the quantitative analysis method come up with the timed prime implicant(PI)that leads to none of actuation demand signal to each of the division of the engineered safety isolation valves.By use of the calculation result of the PIs,gets the reliability index under the AP1000 containment accident ESF actuating,and reaches the purpose of the research and analysis.Through the result of the analysis and research in this paper,it shows that it has a very high degree of reliability for the AP1000 PCS PAMS and ESFAS.And the selection of method of FT and DFM sticks to the needs of the research.The former is suitable for the system dominated by the hardware structure,and the latter is best for the system which is combined by hardware and software with dynamic characteristics.Using different analysis method and get the reasonable research conclusion.The work and achievement in this paper provides an effective way to further investigating the reliability of other AP1000 ESF systems and provide the suggestions for improvement and optimization.