Instrumentation and control (I&C) system is central nervous system of nuclear power plant (NPP), so its reliability is very important for the safety of NPP. Now Digital I&C system (DICS) is widely used in NPP, its reliability should be analyzed more carefully due to the more complex interaction among its components than analog I&C system. The interaction among DICS can be divided into type I and type II. The former is the interaction among sensor failure, controller failure and actuator failure, the latter is the interaction among hardware\software components of DICS. Limited by static Boolean logic, traditional fault tree is hard to model the dynamic interaction among DICS, so dynamic modelling method is required. In this paper, Dynamic flowgraph methodology (DFM) is used for reliability analysis of a subsystem of NPP DICS, namely APC (Automatic Power Control System). The proposed dynamic flowgraph modelled type I and type II interaction among APC and reliability of APC is assessed via inductive and deductive analysis. In the inductive analysis, three basic events (user defined combination of states) are considered, which includes sensor failure, SRB failure and master\checker processor failure, and the sequence of events caused by these basic events are tracked. In the deductive analysis, top event (combinations of possible system parameters and/or component states) is defined as actual control rod position is lower than demanded, and four combinations of basic events are tracked according to occurrence probability of their corresponding event.

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