IRSN is the French national public expert on nuclear and radiological risks and contributes to public policies in the fields of nuclear safety and ionizing radiation protection for public health and environment. In case of incident or accident involving ionizing radiations source term from a nuclear installation, IRSN is missioned to propose technical, sanitary and medical measures aiming to protect the population, workers and environment, and to restore installation safety. To achieve this mission, IRSN performs, from its Technical Emergency Centre, a real-time evaluation of the risks induced by the accidental situation and, if needed, computes expected radiological consequences, taking into account meteorological forecasts. Based on this technical expertise, and combined with local radiological monitoring (Emergency Mobile Unit), IRSN aims to support public authorities and medical/health organizations towards the best decisions (sheltering zone, evacuation zone…) After a short description of the generic assessment method applied at IRSN during a crisis situation, we focus hereafter on the use of precomputed accident type forms at the very beginning of an accident, namely the first hour after the initiation event. Moreover, we present the on-going construction of a scenario database at IRSN, and the related planned developments.