The ETSON (European Technical Safety Organizations Network) PSA Expert Group EG8 has recently considered the insights gained and lessons learned on, from, and for probabilistic safety assessment (PSAa) that Technical Safety Organizations (TSOs) have either conducted or reviewed.
Initiating events and resulting sequences that may occur for design basis accidents (DBA) as well as for design extension conditions (DEC) can be identified by different techniques, typically deterministically by postulating certain failures or combi-nations thereof, but also by probabilistic analyses, usually as a complement to deterministic ones. The added value PSA provides is the ability to rank the relative risk contribution of different systems or components and even the relative contribution of events as well as safety ranking based on quantitative risk figures. The results of PSA can be also used in other safety demonstrations, e.g. for improving the plant safety efficiently or for demonstrating compliance with quantitative safety goals.
Although the maturity and completeness of PSA models have evolved, some events observed from the operating experience might not yet be adequately addressed by current PSA models. Even if PSA models incorporate recent operating experience, the possibility remains that other events could occur which again might not be properly represented in the model. PSA has become indispensable in the current safety culture, and some outstanding achievements of PSA have been identified. Nevertheless, the importance of PSA in decision making differs among member countries and organizations.
This paper summarizes some key points raised by the ETSON EG8 members from their work on PSA, mainly Levels 1 and 2 but also Level 3. These include insights from PSA applications in ETSON member countries as well as from reviews conduct-ed by member TSOs, not only in the framework of regular, full scope PSA but also by case-specific, limited assessments per-formed. The benefits and limitations of PSA, from a TSO viewpoint, are briefly summarized as well. Experience from recent full-scope Level 3 PSAs for new build projects is discussed as well as issues concerned with traditional Level 1 and Level 2 risk metrics being applied to multi-unit sites and SMRs (Small Modular Reactors) and whether Level 3 PSA site-level metrics could provide a solution.