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 76% of the total electric production in 2015. With the increase in intermittent renewable sources, nuclear flexibility is examined as part of the solution to balance electricity supply and demand. Our proposed methodology involves designing scenarios with nuclear and intermittent 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 intermittent renewables (down to load factors of 40% for proactive assumptions). However, the production cost increase could be compensated by progressively replacing the plants: Moreover, incentives are necessary if nuclear is to compete with combined-cycle gas turbines as its alternative backup option. In order to reconcile the social planner with plant operator goals, the solution could be to find new outlets rather than reducing nuclear load factors. Nuclear flexibility could then be considered in terms of using its power to produce heat or hydrogen. (C) 2016 Elsevier Ltd. All rights reserved.