The observation and analysis of the activity of learners in computerized environments training is a major is- sue, particularly in the context of professional training on nuclear power plant full-scope simulator. In such a context, the role of the trainers is critic and require constant alertness throughout the simulation especially for the young trainers. The objective of our work is to propose an approach to facilitate the observation and analysis of the trainees’ activities. This approach is based on interaction traces. It consists in representing the operators’ actions and the simulation data in the form of modelled traces. These modelled traces are trans- formed in order to extract higher informations levels on the behaviour of trainees. Trainers can visualize the different levels to analyse the reasons, of successes or failure of trainees. This approach has been implemented in a prototype, called D3KODE, allowing the representation, processing and visualization of traces. D3KODE was evaluate according to a comparative protocol conducted with a team of trainers from EDF Group.