Study On A Reliability Analysis Method Of Digital Instrument & Control System At Nuclear Power Plant Based On Multilevel Flow Model

The Instrumentation & Control (I&C) System is the central part of the nuclear power plant (NPP) and crucial to the safety operation of NPP. Among the NPPs under operation, the digital I&C (DI&C) system is gradually taking place of the analogue I&C (AI&C) system.Meanwhile, DI&C technique has been fully integrated into the design of the NPP project either under construction or planning. Compared with the AI&C technique, DI&C technique has obvious advantages in logic processing, calculation capacity, accuracy of the measurement and control, etc. However, it accompanied with the issue of the reliability assessment and verification of DI&C system due to its special structure, multi-state and dynamic characteristics. Up to the present, there isn't any approach which could complete the reliability assessment of DI&C system as a widely acceptable one.This thesis proposed an approach of reliability modeling and analysis to DI&C system based on Multilevel Flow Model (MFM). The main work is as follows:(1) Discuss satisfaction condition of dynamic methods using on DI&C system and propose MFM as the technical solution.(2) Select a digital feedwater control system (DFWCS) as the study case: establish the process system model then establish the two interactions model with control according to the control rules and two kinds of dynamic interaction characteristics.(3) Carry out the reliability analysis of DFWCS by fault tree (FT) technique based on failure mode and effect analysis. Discretize the function state and build the function decision table firstly. Then build the sub-FT and connect them into FT. Finally analyze the FT and make the post-treatment to the result in view of the functional mult-state until obtaining the minimal prime implicant.(4) Select the PID module in DFWCS. Establish the MFM models according to the software structure and obtain the unit failure probability by testing. Complete the model by using the developed modeling analysis tool and finish calculating the general software reliability through the unit reliability updated by testing. Then achieve the goal of software quantitative assessment.In summary, this thesis carries out the reliability modeling and analysis to DI&C system from an overall perspective, completes the study on MFM method applied to DI&C system reliability analysis at NPP and lays a technical foundation for the further work.