In France, 27% of the electricity is to be produced by renewable resources by 2020. This share is intended to grow continuously up to 2050. The recent European agreement and the French energy transition law will promote such a development. The French power system is characterized by high nuclear penetration and nuclear power is meant to remain a significant contributor in the medium and long term, as a low-carbon power source. More than half the French nuclear power fleet was installed in the late seventies / early eighties. Thus, the issue of its replacement is at the core of the French power mix issue. The objective of this paper is to provide some insights about the opportunity it enables for the energy mix. Two plausible replacement scenarios are developed and analyzed as regards the energy cost provided by nuclear power. For a given target level of nuclear installed capacities, the penetration of non-dispatchable renewables with dispatch priority will increase the need for nuclear power modulation at reduced average load factor. The impact of modulation on the nuclear levelized cost of electricity is assessed, according to the considered replacement scenario and for different renewable and nuclear energy penetration scenarios. Results show that, according to the selected assumptions, implementing a progressive shut-down (based on an increased operation lifetime of Nuclear Power Plants) appears a relevant choice since it both provides a lowest power production cost even at reduced average load factor to participate to load following and allows the possibility of waiting for choosing most sustainable technologies.