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MERMOS is the reference method used by Electricite de France (EDF) for Human Reliability Assessment, to assess the operation of nuclear reactors during incidents and accidents for Probabilistic Safety Evaluations. This method, like the other “second generation” methods, is not easy to understand, since it involves complex concepts : however, its results are easier to understand and use. Thus this paper approaches the MERMOS method using a “bottom-up” approach, starting with the method’s results. MERMOS models the failure of a mission involving a “human factor” by describing qualitatively all the ways of failing, as a number of little stories ; it does not present a list of factors affecting the failure of the mission without defining the means by which they exercise their effect. Of course, the format of these short stories (or MERMOS “scenarios”) is defined very strictly, to take into account as far as possible knowledge of the human sciences (ergonomics, psychology, sociology and accidentology) and of techniques (operational safety, probability studies and the operation of nuclear plants during accidents). We shall describe these little stories using an example (non-nuclear) to explain firstly how they are structured to allow realism and rigour ; and secondly which human-reliability concepts they use (modelling systems for behaviour during an accident, with reconfiguration and CICA (Important Configuration of Emergency Operation). A little story that leads to the failure of an HRA mission describes how a system may continue too long in a direction (or configuration) that proves inappropriate (or no longer appropriate) to the situation.