On Reliability Assessment Methodologies Of Functional Safety Digital Control System For Nuclear Power Plant Based On Markov Modeling

As the reliability of digital control system affects the safety and availability of nuclear power plant, this paper discuss the reliability assessment methodologies for functional safety digital control system of nuclear island. There are many methods used to evaluate the reliability of digital instrumentation and control system. But none is recognized by technical community of nuclear industry, and there is not a set of uniform standards for reliability assessment of digital instrumentation and control system of nuclear power plant. Both the dynamic interactions between the reactor protection and control systems and controlled plant physical processes, and the dynamic interactions between the components of the reactor protection and control systems itself should be accounted for, When assessing the reliability of digital instrumentation and control system of nuclear power plant. The fault tree model, which is static analysis method used in reliability assessment of digital control system in current nuclear power plant, can describe the interactions between the reactor protection and control systems and controlled plant physical processes. But it cannot be able to model the dynamic character of the system. This paper evaluates the reliability of functional safety digital control system of nuclear island using Markov modeling.Details are as follows:1.Analyze the function and overall architecture of digital instrumentation and control system in nuclear power plant, the architecture of protection system, and the architecture, character, operation mode and flow of Functional safety digital control system of nuclear island.2.Discuss the Failure Mode of the components of Functional safety digital control system of nuclear island, such as analog input module, main controller, back up controller, power module, digital output module, and so on.3.Build the Markov modeling of Functional safety digital control system of nuclear island,calculate availability ratio and MTTF of the system based on the Markov modeling. It shows that the Markov modeling have the ability to provide relevant Information to users.4.Discuss the common cause failures of controllers, build the Markov modeling of Functional safety digital control system of nuclear island considering common cause failures,calculate availability ratio and MTTF of the system based on the Markov modeling. Common cause failures should be considered, during evaluating the reliability of the system5.Discuss the difference between the Markov model and fault tree model, which is static analysis method used in reliability assessment of digital control system in current nuclear power plant. Compared with the fault tree analysis method, Markov modeling has the following advantages: 1) Various reliability date, such as availability ratio and MTTF of the system, can be calculated from the same Markov modeling.2) Various state, such as normal operation, degrade operation and failure, can be described by the same Markov modeling, and Various factors refer to the reliability can be Analyzed by the same Markov modeling.3) Markov modeling can be able to model the dynamic character of the system. The comparison shows that the reliability date calculated from the Markov modeling is correct, and Markov modeling can be used to evaluate the reliability of the Functional safety digital control system of nuclear island.