Applying H-STPA on a nuclear I&C system

Instrumentation and control (I&C) systems are the central nervous system of nuclear power plants (NPPs), governing all operational states. With analog systems becoming obsolete, modernizing digital I&C systems is essential but introduces challenges such as complex human-machine interactions. System-Theoretic Process Analysis (STPA) and its extension, “Engineering for Humans” (H-STPA), have been recognized as suitable methodologies for addressing these challenges, as they integrate technical and human control actions within a unified framework. This study applies H-STPA to the feedwater control system of a Finnish NPP, focusing on two manual control actions and their associated unsafe control actions (UCAs). We adapted France’s H-STPA into an interview-based approach with experienced operators to uncover reasons behind unsafe behaviors. The results show that H-STPA enhances traditional STPA results by generating richer, context-specific loss scenarios. For two control actions, we identified 15 additional, detailed loss scenarios. Our findings provide evidence of H-STPA’s value in NPP modernization, particularly for obtaining human operator knowledge and refining safety requirements. Future research should explore optimal strategies for applying H-STPA, including narrative-based techniques and methods to increase practitioner awareness of human decision-making.