[en] The nuclear safety approach imposes to take into account in the design of a reactor, a list of events and configurations that are considered as precursors of transients important for the safety of the reactor. These incidental or accidental transients should be thoroughly studied during the design phase in order to be sure that safety criteria are well respected with a sufficient margin. These studies are framed by procedures that are based on sets of input data, on simulation models and on computer codes and their validation domain. The operating life of a study procedure is usually over 10 years. A lot of procedures that date back in the seventies rely on simplified models and appear to be too conservative to today's standard. The generalization of 3-dimensional calculations for reactor core codes and the coupling between neutron transport codes and thermalhydraulic codes make necessary a progressive updating of the study procedures. (A.C.)