Abstract: Background: Redundancy configuration is widely adopted in the digitalized instrumentation and control (I&C) system of third-generation reactors. For further availability improvement, state monitoring method of redundant configured Programmable Logic Controller (PLCs) and Input/Output Controller (IOC) components in Experimental Physics and Industrial Control System (EPICS) based I&C system prototype was studied for the Thorium Molten Salt Reactor (TMSR) project. Purpose: In order to avoid serious consequence or potential risks caused by switch-over or redundancy failure, the running states of redundant units composed high availability (HA) devices in reactor I&C system are expected to be accessed in real-time in this study. Methods: By means of adding the unit state reporting channels which could be transferred into Process Variables (PVs) for the acquisition of EPICS upper tier application, a novel method of monitoring redundant states of device units was developed. Source code of redundant monitor task (RMT) was modified by adding a routine to record state-related information for the IOC pairs whilst a new state-read program block for local and remote units was developed for PLCs. All state monitoring related variables was acquired and passed into EPICS records by IOC driver. In terminal layer, a CSS (Control System Studio)-based OPI (Operator Interface) Plug-In was implemented for on-line state monitoring. Results: Redundant state-related information of each unit pair were successfully acquired as PV values and showed in the designed OPI interface by proposed method. Test proved it could achieve real-time monitoring of redundant state variation. Conclusion: By developing and modifying program for HA units, and designing OPI interface, reactor I&C system could monitor redundant unit state for a long-term with few additional system resources. It could provide technological support for further improvement of availability of the reactor I&C system.

Key words: Redundancy, Distributed Control Systems (DCS), Experimental Physics and Industrial Control System (EPICS), High availability, State monitoring