The recent occurrence of severe major accidents has brought to light flaws and limitations of hazard identification (HAZID) processes performed for safety reports, as in the accidents at Toulouse (France) and Buncefield (UK), where the accident scenarios that occurred were not captured by HAZID techniques. This study focuses on this type of atypical accident scenario deviating from normal expectations. The main purpose is to analyze the examples of atypical accidents mentioned and to attempt to identify them through the application of a well-known methodology such as the bow-tie analysis. To these aims, the concept of atypical event is accurately defined. Early warnings, causes, consequences, and occurrence mechanisms of the specific events are widely studied and general failures of risk assessment, management, and governance isolated. These activities contribute to outline a set of targeted recommendations, addressing transversal common deficiencies and also demonstrating how a better management of knowledge from the study of past events can support future risk assessment processes in the identification of atypical accident scenarios. Thus, a new methodology is not suggested; rather, a specific approach coordinating a more effective use of experience and available information is described, to suggest that lessons to be learned from past accidents can be effectively translated into actions of prevention.

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International audienceProper hazard identification (HAZID) in safety reports has become progressively more difficult to achieve. Several major accidents in Europe in recent years, such as Buncefield and Toulouse, were not even considered by their site 'Seveso-II' Safety Case. One of the reasons is that available HAZID methodologies take no notice of apparently least likely events. Nonidentified scenarios thus constitute a latent risk, whose management is extremely complex and open ended. For this reason, the EC project iNTeg-Risk, in one of its tasks, aimed to investigate the issue of atypical scenarios and explain how they could have been identified. This study wants to describe the approach used and its immediate results, paving the way towards a new method for the identification of atypical accident scenarios. An in-depth accident analysis of some of these accidents was performed, in order to outline general features of plants in which they occurred, their causes, consequences, and lessons learned. This analysis followed a precise common scheme, which allowed a systematic approach to the problem by the experts involved. Based on the findings, failures connected to risk management and risk appraisal were identified. Three main basic issues in risk appraisal were identified: the low perception of emerging risks related to atypical accident scenarios, the lack of knowledge about related events, such as early warnings, and the incapability of current techniques in leading analysts to the identification of atypical scenarios