In France, during nuclear emergency situations, the Institute for Radioprotection and Nuclear Safety (IRSN) provides technical, public health and medical guidance to French public authorities on measures to be taken to protect the population, workers and the environment, while contributing to the restoration of the safety of the facilities. IRSN has an emergency response center where emergency responders are mobilized to assess the emergency situation in several technical units, one of which is dedicated to the assessment of the facility. The main tasks of this "facility assessment" unit are to assess the current state of the damaged "object", predict the development of the emergency situation, evaluate the radioactive materials releases into the environment, and verify the suitability of the operator's measures to minimize the releases and retrieve a stabilized and safe state of the facility. Within this unit, there is a "criticality specialist", whose function is to assess an ongoing or ended criticality accident but also to evaluate the criticality risk in any case of an emergency situation. After a brief overview of IRSN's emergency organization, this article details how the "criticality specialist" function fits into this organization and in which cases it is activated. The article also presents the tools and resources used to fulfil this function, how the "criticality specialists" are recruited and trained, and how they practice to ensure that they are operational during emergency situations. A first experience feedback acquired by this function during its participation in drills and real emergencies is also shared, as well as the resulting perspectives.