Simulation exercises have become a common feature of crisis preparedness in public and private organisations. The analyses of these exercises have however rarely examined the production of the scenarios on which these exercises are based. Yet the observation of exercises conducted in the field of civil nuclear production in France suggests that far from placing participants in an unexpected and uncertain environment scenarios tend to promote an orderly representation of crisis situations. This is found to derive from the multiple constraints and framings that weigh on the scenarios writing process which are themselves the results of tacit agreements and power relations existing among the different organisations in the civilian nuclear sector (nuclear operators, regulators, experts) and civil security.