Precursors and startling lessons: Statistical analysis of 1250 events with safety significance from the civil nuclear sector

Highlights: • "Reactive" reporting triggered after major accidents. • Leading causes of multi-unit events are external triggers and design issues. • Design, procedural, and maintenance errors are leading contributors to CCFs. • Aging signs observed after 25 years of operation. • Emphasizing vulnerabilities and identifying precursors to major accidents. We analyze the ETH Zurich open curated database of 1250 worldwide nuclear events focused on safety significance with potentials for precursors, presented in the companion paper. We find that major accidents always trigger a wave of "reactive" reporting as well as changes in regulatory or corporate management that last 5 to 6 years, mostly due to increased alertness, improved transparency, uncovering latent design errors, and heightened public pressure. The leading causes for multi-unit events are found to be external triggers and design issues, confirming the need to adapt PSAs to cover multi-unit events accordingly. Common-cause failures (CCF) are found to occur fairly frequently, at different levels, and can significantly erode the safety of the plant. From the lessons learned from this analysis, we suggest that frequent review of components design and operating procedures, employing different teams for testing and maintenance activities on redundant trains, and sharing operational experience between plants of similar designs, are some of the steps that should be taken in order to limit future occurrences of CCFs and beyond that further improve plant safety. We identify some quantitative signs of aging for plants after the age of 25. Our findings stress the need for larger recording, reliance, and sharing of operational data to support learning from experience and avoid reoccurrence of accidents and events.