STPA-based approach to identifying extraneous errors of commission in FOAK advanced reactors

First-of-a-kind (FOAK) advanced nuclear reactors, including small modular reactors, increasingly rely on autonomous safety systems to achieve high levels of safety and efficiency. While such designs reduce the need for manual intervention, they also introduce new complexities in supervisory control that may lead to errors of commission (EOCs). In particular, extraneous EOCs, referring to operator actions that are unnecessary and potentially hazardous, have not been systematically addressed by conventional human reliability analysis methods. This study investigates the feasibility of applying Systems-Theoretic Process Analysis (STPA) to systematically identify extraneous EOCs in autonomous safety contexts. An application to a fail-safe passive safety system demonstrates how STPA can trace the causal mechanisms of inappropriate operator interventions, revealing loss scenarios and pathways specific to extraneous EOCs. Beyond this methodological validation, the study derives example requirements to support safe interaction between autonomous safety functions and human operators. These requirements illustrate how STPA results can be translated into general guidance for ensuring that human–automation interactions preserve system safety while accommodating supervisory flexibility. In doing so, the proposed approach extends the analytical scope of human reliability methods and provides a structured foundation for managing operator–system dynamics in FOAK advanced reactors.