Research On Realtime Automatically Updatable Living PSA Methodology For NPPs

Risk monitoring is the process of evaluating the risk of a Nuclear Power Plant(NPP)unit in different configurations during its lifetime.The results can be applied to risk-informed safety management in NPPs,to balance the safety and economy under the precondition of ensuring nuclear safety.The Living Probabilistic Safety Analysis(Living PSA)method is an approved and widely used modeling and analyzing method to implement risk monitoring in industry,and many software tools(named as Risk Monitor,RM)have been developed based on Living PSA.Existing RMs are mainly used for off-line applications,such as of maintenance plan optimization and risk assessment after system modification.The assessment is offline,static,and initial(usually not conservative).As the technique development of information system and digital instrumentation&control system in NPPs in recent years,it is implementable to continuously collect equipment status information(operation,standby,failed,in maintenance,etc.)during the NPPs operation.It provides the foundation of online risk monitoring,which is to evaluate the risk of an NPP unit continuously and timely at any time point during plant operating according to the current plant configurations(component status in operation systems and safety-related systems).Instantaneous risk that continuously change with time and the real configurations of units can be evaluated by online risk monitoring.The development of risk monitors from off-line application to online application has led to higher requirments on the accuracy of Living PSA.Event Tree(ET)and Fault Tree(FT)are the two basic elements of PSA.Therefore,the update strategies of event tree and fault tree are studied separately first.And it is found that Event Tree updating can be implemented through Fault Tree updating.Then,research on Living PSA is focused on Fault Tree modeling and updating.And also the technical framework and key issues to be studied for Living PSA methodology are determined.According to the possible changes of Nuclear Power Plants during operation,the following improvements have been made to the existing Living PSA method:1)Time-dependent and status-related Basic Event(BE)probability models are proposed to involve possible factors which may affect the reliability and importance of a component in consideration,including the current status,demand status and operating history.Then,the correspondence relationship among the BE models and equipment states is established,by which the BE models can be updated automatically.2)Research is carried out on widely used Common Cause Failure(CCF)parameter models to find out how to implement the parameter updating when CCF group order is reduced.The theoretical parameter updating models are obtained,and then the update process and simplified parameter update formulas are suggested according to actual engineering processes.3)To evalute the sequential failure issues in redundant system,the concepts of "Sequence-Conditional Failure" and "Sequence-Dependent Failure" are introduced,and"Conditional-AND" gate and "Priority-AND" gate are constructed respectively,forming the sequential fault tree(SFT)model.The Boolean logic laws for qualitative assessment are extended for SFT and also the Minimum Cut Sets(MCSs)quantification formulas.Two SFT modeling and updating schemes are proposed:One is to enumerate all possible situations in a FT model,and the other is based on computer-aided automatic FT construction.Based on the above research,the functional requirements of Living PSA software for modeling and updating are established,and the software design is carried out,including data structure,workflow and human-machine interface design.By the software developed according to the design,modeling and updating can be emplemented based on the proposed Living PSA methodology.The softwared has been verified by independent third-party and industry experts.Finally,a case study is carried out to demonstrate the comprehensive Living PSA modeling and updating process on NPP Component Cooling Water System,with a set of configuration changes presetted.Research tells that,compared to the existing Living PSA methodology,the proposed method has several advantages as follows:1)System risk and corresponding important risk contributors obtained from the proposed Living PSA are related to not only system configuration but also system operating time.Thus,the risk imformed operation and maintenance management can be carried out more respectively.2)The over optimistic in existing Living PSA is compensated by considering the influences of equipment operation history in BE probability models;3)The suggested methodology for Common Cause Failure Update is more accurate,compared to currently engineering adopted measure in commercial RM tools;and4)The failure paths of sequential failure system obtained from the proposed Living PSA model are more accurate,and also the quantification,eliminating unnecessary conservation in existing Living PSA.And there is evidence that the proposed Living PSA method may find new failure paths which are missed by the existing method.