This phd aims to understand how decommissioning projects deal with the unexpected. On the one hand, the aim is to identify the characteristics of unexpected situations, from the point of view of the activity of the decommissioning players and the issues of work safety and safety of the installations they pose. On the other hand, it is a question of studying the way in which this unexpected situation leads to the mobilisation of certain parts of the organisation (site, installation, site, company), and how this mobilisation shifts the boundaries of the organisation by calling on external players (public expert, safety authority).