[en] To account for successive evolution of nuclear power plants emergency operation, EDF had several times to review PHRA methodologies. It was particularly the case when event-based procedures were left behind to the benefit of state-based procedures. A more recent updating was necessary to get pieces of information on the new unit type N4 safety. The extent of changes in operation for this unit type (especially the computerization of both the control room and the procedures) required to deeply rethink existing PHRA methods. It also seemed necessary to - more explicitly than in the past - base the design of methods on concepts evolved in human sciences. These are the main ambitions of the project named MERMOS that started in 1996. The design effort for a new PHRA method is carried out by a multidisciplinary team involving reliability engineers, psychologists and ergonomists. An independent expert is in charge of project review. The method, considered as the analysis tool dedicated to PHRA analysts, is one of the two outcomes of the project. The other one is the formalization of the design approach for the method, aimed at a good appropriation of the method by the analysts. EDF's specificity in the field of PHRA and more generally PSA is that the method is not used by the designers but by analysts. Keeping track of the approach is also meant to guarantee its transposition to other EDF unit types such as 900 or 1300 MW PWR. The PHRA method is based upon a model of emergency operation called 'SAD model'. The formalization effort of the design approach lead to clarify and justify it. The model describes and explains both functioning and dys-functioning of emergency operation in PSA scenarios. It combines a systemic approach and what is called distributed cognition in cognitive sciences. Collective aspects are considered as an important feature in explaining phenomena under study in operation dys-functioning. The PHRA method is to be operational early next year (1998). Preliminary validation tests were performed quite early in the design process. These tests will set the grounds for the presentation of examples of the application of the method. (authors)