The complementary features of low-carbon power sources are a central issue in designing energy transition policies. The French current electricity mix is characterised by a high share of nuclear power which equalled 73% of the total electric production in 2013. With the increase of non-dispatchable renewable resources, nuclear flexibility is examined as part of the solution to balance electricity supply and demand. Our proposed methodology involves designing scenarios of nuclear and non-dispatchable renewable penetration levels, and developing residual load duration curves in each case. The load modulation impact on the nuclear production cost is estimated. This article shows to which extent the nuclear annual energy production will decrease with high shares of non-dispatchable renewables (down to load factors of 40% for proactive assumptions). However, the production cost increase could be compensated by implementing a progressive replacement of the plants. Besides, incentives proves required for nuclear to compete with combined cycle gas turbines as its alternative backup option. In order to make the social planner and plant operator goals coincide, the solution could be to find new outlets rather than reducing nuclear load factors. To conclude, nuclear flexibility could then be considered through the power use by producing heat or hydrogen.