Passive Systems Performance and Reliability Workshop attracted around 150 experts, including individuals representing regulators, technical support organisations (TSOs), research institutes, vendors and international organisations.
The Nuclear Energy Agency (NEA), in collaboration with the European project, “Ensuring Assessment of Safety Innovation for SMR", EASI‑SMR, and support from the International Atomic Energy Agency (IAEA), held the event in Paris, France, from 30 March to 1 April 2026.
Passive safety systems play a central role in the safety concepts of many advanced reactor designs, including small modular reactors (SMRs) and micro reactors. At the same time, international experience shows that broad consensus is still evolving on modelling approaches, acceptance criteria, and methods to credit passive system performance and reliability in safety analyses. The workshop programme supported exchange across communities and methods.
On the first day, participants heard industry and regulatory perspectives, including sessions on international initiatives and licensing experience. Followed by a second day featuring technical sessions and breakout discussions.
The workshop ended with a dedicated wrap‑up session consolidating feedback from the technical sessions and discussing a shared roadmap for future work. After the close of the workshop, select individuals participated in an optional technical visit to EDF facilities.
Discussions across sessions converged on several key messages. For one, demonstrating reliability should be central to success, and must be evidence-based.
Participants noted a broad shift from demonstrating design feasibility to providing a structured and traceable reliability demonstration, grounded in experimental evidence, validated methods and explicit consideration of failure mechanisms and modes. The importance of appropriate failure modelling and periodic testing strategies for passive systems was emphasised in the context of licensing confidence.
Experimental programmes, and verification and validation (V&V) were also identified as critical. The workshop highlighted the role of integral and separate effect testing, scaling considerations, and V&V activities in establishing confidence in passive system performance. Participants also pointed to persistent constraints, notably the high cost of large experiments and advanced instrumentation, and the resulting limitations on data availability, particularly for first‑of‑a‑kind systems and long‑mission scenarios.
Discussions underscored the need to integrate deterministic safety analysis (DSA) insights, uncertainty treatment and empirical evidence into probabilistic safety assessment (PSA) approaches for passive systems, including the treatment of common‑cause and dependent failure mechanisms.
A recurring point was that passive systems do not remove the human role; rather, they can shift it toward monitoring, diagnosis, verification and recovery. Human factors therefore extend beyond the control room and must be addressed at the plant‑system level, including interfaces and operational strategies.
Participants highlighted the potential benefits of early regulator interactions and pre‑assessment activities, particularly where markets and design contexts are similar, while maintaining the need for robust, well‑documented evidence bases and clear acceptance frameworks. The workshop helped facilitate multidisciplinary exchange among experimentalists, code developers and safety analysts; addressed modelling challenges and acceptance criteria; and (aided in identifying knowledge and data gaps from both thermal‑hydraulic and probabilistic perspectives.
A workshop report will be published summarising findings and recommendations for future R&D; proceedings will also be published by the EASI‑SMR project.
