In order to maintain competence or to train new employees, it is necessary to prepare and organize some training sessions suitable to the concerned audience. As a TSO (Technical and Scientific Support Organization), IRSN is particularly implicated in this issue, specifically in the field of human radiation protection, protection of the environment and nuclear safety. IRSN created recently an “In-house University” containing different “Schools”: “Assessment”,“Emergency planning/Response”.... The main objective is to support IRSN employees following a specific route. Concerning the “Assessment” school, this route is first composed of a training on general topics (e.g. “how to perform an assessment?”), then of other trainings on specific topics (“nuclear criticality safety”, “fire risks”, “containement”...). Implemented teaching methods are innovative and less academic than it used to be. These methods have been taught by training professionals specialized in andragogy (methods and principles used in adult education). In this context, the “nuclear criticality safety” (NCS) training has been completely renewed by the SNC department (Neutronics and Criticality Safety Department)of IRSN. Scheduled over a period of one week, each day an expert teacher stays with the participants and interacts with the other speakers planned in the agenda, which allows dynamic discussions. Moreover, various exercises in groups (using paper boards, post-its, videos ...) give the opportunity to test the newly acquired knowledge. The session begins with general notions concerning NCS (physical properties, consequences of a nuclear criticality accident ...). Then, each control mode (mass, geometry ...) is discussed considering lots of examples from actual facilities. Finally, a review of past nuclear criticality accidents is done, and the impact of other risks (fire, flood, ...) on NCS is evaluated. A future project is to propose an international training in English intended in particular for other TSOs via the ENSTTI organization (European Nuclear Safety Training and Tutoring Institute).