Electricity production in France relies mainly on nuclear energy and is one of the most decarbonized in the world, emitting approximately 50 g/kWh. This production is ensured by 56 pressurized water nuclear reactors commissioned between 1978 and 1999. Extending the lifespan of these reactors is crucial to guarantee electricity supply in the years to come, especially considering the perspective of increasing electrification of society and potential tensions in European energy supply. The nuclear industry is exploring the possibility of extending the operation of some reactors beyond 50 or even 60 years. This prospect raises questions regarding the aging of components, particularly those in the primary circuit. Among these components, the reactor vessel is of particular importance as it contains the reactor core and is not replaceable. The vessel is made of low-alloy steel (16MND5) and is subject not only to thermal aging but also to irradiation aging. Interfacial segregation of solutes, notably phosphorus, is one of the mechanisms of steel embrittlement. This segregation, which refers to the accumulation, by solid-state diffusion, of solute atoms at grain boundaries of a material, can occur during manufacturing heat treatments as well as during reactor operation. It is in this context that the SIRA chair aims to improve understanding of the aging by segregation of steel components in the primary circuit of nuclear reactors. This initiative responds to a demand from Framatome and EDF, the main players in the nuclear industry in France. Segregation predictions, integrated into the design and operation codes of nuclear equipment, are currently obtained using the so-called "Druce model." This model is based on several simplistic, even unrealistic assumptions, and its predictions are not validated by recent segregation measurements made as part of laboratory aging programs. The work of this chair aims to improve understanding (1) of the effect of initial microstructures, including welding microstructures, on intergranular segregation and associated embrittlement, and (2) of the thermodynamic and kinetic aspects of segregation, taking into account the chemical interactions of phosphorus with alloying elements. Methodological developments are also planned to improve measurements of intergranular segregation by Auger spectroscopy on fracture and by energy-dispersive analysis in transmission electron microscopy (STEM-EDX). The work program will be supported by three CIFRE theses and 116 months of post-doc research. Beyond the scientific aspect, the chair holds strategic importance for the French nuclear industry. The knowledge developed will contribute to component maintenance programs in service, particularly in the perspective of reactor extension, and to the optimization of component manufacturing for new reactors (EPRs). This initiative will also help maintain a level of equipment and skills in both academic and industrial sectors commensurate with the challenges related to component aging.