The Research Of Safety Analysis Method Based On The System Theory Model For Nuclear Power Plant Digital Instrumentation And Control System

The reliability and safety of the instrumentation and control system,which serves as the control center and nerve center,is the fundamental element in normal operation of nuclear power plant.After the use of analogy and distribution instrumentation and control system,the new instrumentation and control system in nuclear power plant take the digital technologies to construct its control system.While the digitization technologies improve the safety and reliability,it also brings new failure modes to the system.In digital instrumentation and control system,software becomes the key components of the system.The malfunction model of software differs from that of the hardware.When design and operat the control system,it needs to consider the coordination efficiency between software,hardware,environment and manual control.The traditional safety analysis methods in nuclear power plant are based on reliability theory,such as FTA,FMEA and etc.The goal of these methods is to find the components failures and take them as bottom events.This kind of domino modeling analysis doesn't fit into system containing indirect and nonlinear relationship between events.Because the systematic complexity of digital instrumentation and control system in nuclear power plant improves a lot due to the use of computer and other new technologies,human cannot predict,identify and defense those inter-behaviors among all system components.These kinds safety failure caused by unknown inter behavior is not actual failures of components but caused by unintended inter-behavior between components and the incorrect system specification.Traditional analysis method cannot solve this problem.Researchers proposed some safety analysis methods based on systematic theory from a system-based angle,including Rasmussen's Hierarchical Model of Socio-Technical System,Hollnage's Function Resonance Accident Model(FRAM)and Leveson's System-Theoretic Accident Modeling and Processes(STAMP).STAMP model considers the cause of accidents is not be purely caused by components failures,it can also be triggered by abnormal interbehvaior between components.To ensure system safety,we need to impose correct mechanism to these unpredicted and abnormal inter-behaviors.Therefore,we can identify the unsafe control action and causal factors based on the system control structure and prevent the occurrence of safety accidents by enforce the proper constraints on the unsafe control actions.By taking STAMP model as the theory basis and STPA(Systems Theoretic Process Analysis)method as application tool,the paper conduct a safety analysis and evaluation on the Chinese national safety instrumentation system and control and take the reactor emergency trip subsystem as analytical object.It analyzes the causes of system failures under automation procedures and manual control in detail.Then it applies safety analysis to the reactor emergency trip subsystem by taking the trip failure as the top event and separately applying FMEA and FTA safety analysis method to the case.Through comparison on safety analysis and quantization calculation results of those three methods,this paper comes up with their advantage and shortcome,features and applicable environment.We also point out that there are still some problems to slove in applying STPA analysis to digital control system in nuclear power plant.In order to provide the ability to describe the multi-redundancy structure and to get the quantitative results,this thesis propose an new method which more suitable for nuclear digital control system:NMSIE-STPA.This method adopts the boolean logical formula and the logical deduction method to depict the multi-redundancy structure and explain how to get the minimum safety events sets.Inspired by the structure importance index in FTA,the NMSIE-STPA method presents a safety influence effect index and a computing method to extend the results of STPA method.These expansions make the results be more valuable and pretty suggestive.The innovation of the thesis is as follows:(1)We introduce a new safety analysis method which based on system theory and apply it on nuclear digital control domain.The study identifies some new underlying safety accident causes and proposes corresponding measures.At last,we provide a new style safety analysis report.(2)The study explores the application of the three safety analysis methods: FMEA,FTA and STPA.According to the analysis results,we give a quantitation comparison table.The comparsion shows that the STPA method has some advantages over the other two.At last,we point out the selecting principle of the safety analysis methods.(3)According to the STPA method analysis results,the study proposed that the Boolean logic representing can be used to depict the Multi-redundant physical structure which used in digital control system widely.Meanwhile,the study gives a new way to generate a minimal safety event set for STPA method.(4)Makes reference with the importance concept in reliability theory,the study proposed a new safety indicatior which named as safety influence effect.Used this indicator,we compare and rank the safety influence effect of the safety basic event in a quantiation way.At last,we provide a creative computational method of safety influence effect which refers the reliability calculation method to get the rank results.