Safety Analysis of Signal Quality Bits in Nuclear Power Plant Distributed Control Systems Based on System-Theoretic Process Analysis Method

As a part of self-diagnosis functions, the instrumentation signal quality bit (SQB) is widely introduced in engineering applications, especially in nuclear power plant (NPP) distributed control systems (DCSs). However, the introduction of SQB significantly increases the DCS software complexity and may potentially be unsafe against its design intention. To reduce unscheduled shutdown accidents caused by potential unsafety, it is necessary to conduct a systematic safety analysis on the SQB. Therefore, this paper selects a CPR1000 NPP unscheduled shutdown accident as the research object to abstractly model and reproduce. The system-theoretic process analysis method is applied to analyze the SQBs involved in the unscheduled shutdown accident, which provides a way to identify the behavior of the system components, the interactions between them, and the safety constraints from the perspective of the whole NPP. Therefore, the weaknesses and potential factors affecting safe NPP operation are located, and countermeasures can be proposed. The safety analysis process can inform the identification of potentially unsafe scenarios and complement the verification and validation of the DCS software. Finally, the NPP economy should be improved while maintaining its reliable and safe operation.