Risk-based precursor analysis essentially consists of the mapping of an operational event — this is a safety-related incident that occurred in an industrial installation like a nuclear power plant (NPP) — on a risk model of the installation. This risk model might be a full probabilistic safety analysis (PSA), which is available now for all NPPs. The analyses may result in a more objective risk ranking of safety-significant events, called accident precursors, as to their possible consequences. By asking what-if questions, the study might also involve still other potential accident scenarios, based on the real scenario as it happened, to be considered for operational feedback. The obtained ranking might help to prioritise corrective actions, and — in case of a full precursor analysis program — to monitor and to trend the safety level of the industry.

In a few countries, such precursor studies were already practiced since the eighties. In the late nineties, AVN — a nuclear regulatory organisation in Belgium — was involved in an international project to develop methodological guidelines for PSA-based event analysis (PSAEA). After two pilot studies on incidents that occurred in Belgian NPPs, AVN decided to run a PSAEA program with the objective to determine the quantitative importance of a few well-selected operational events per year, and — if significant — to identify possible safety issues for improvement. Besides the global safety evaluation of the NPPs, it constitutes the first PSA application in Belgium. To date, about 25 Belgian events have been analysed in detail. They have been presented to the utility, if applicable with a request for further follow-up. Today, the PSAEA process has become an integrated, risk-informed part of the overall AVN process of follow-up of operating experience. The paper gives some examples and elaborates on an overview of experiences with these studies.

In order to ensure a close contact with other experiences and developments in this field, AVN has taken the initiative to organise an annual meeting on probabilistic precursor analysis in the nuclear industry. The interaction with a wide audience of other practitioners and stakeholders at this forum has not only advanced the understanding of many technical issues, but it has also contributed to the evolution of a broader view on the process itself of precursor analysis by a nuclear regulatory organisation or by a utility. Ever since, eight international technical meetings as well as an OECD/NEA workshop have been organised by AVN on this topic. The paper makes an attempt to summarise the various activities worldwide in this field, as they were reported at this forum. It elaborates on some similarities and differences in various aspects, such as objectives, screening, analysis and modelling methodology, results, uses and achievements. Moreover, areas for further improvement are addressed.