Modulation in the French nuclear fleet, and the consequent gap between nuclear availability and output, is set to grow in the coming weeks as consumption falls in summer and solar output picks up.
But the fleet's ability to modulate, touted as one of its great strengths, could be put to the test by growing amounts of intermittent renewable capacity, without any accompanying rise in consumption or in flexible storage capacity.
France's flexible nuclear plants are unusual in being able to modulate their output downwards, with each reactor capable of dropping twice a day to 20pc of rated output.
Nuclear reactors by their nature have high fixed costs and low variable costs. But operator EdF still reduces production in hours in which prices fall below these low variable costs, widening the gap between the theoretical available capacity and actual production. And low-priced hours became more common last year, particularly in the summer, in the middle of the day and on weekends, as low demand coincided with high nuclear and renewable production.
The gap between availability and output across the fleet typically has held at 1-2GW on a monthly basis in recent years. But last summer, it jumped to 4GW on average in each month from April-August (see availability-output gap graph).
And so far this year it has averaged 2GW, compared with 1.8GW in the same period last year. Modulation has held higher too, with the difference between maximum and minimum daily nuclear output averaging 4.6GW, up from 3.2GW last year (see modulation graph).
Solar output so far this year has averaged 2.4GW, up by 35pc on 1.8GW in the same period in 2024, after France's solar fleet grew by 5GW, or roughly a quarter, over 2024. As output increases with longer days, this will begin translating into increasingly more output centred around midday, driving stronger modulation.
Modulation becomes political
Modulation has this year become a political football, with the government promoting the parallel growth of nuclear and intermittent renewables, while an insurgent faction, typically on the political right, claims that more renewables are at best wasteful and at worst actively damage the nuclear fleet.
France's nuclear fleet has always modulated, as its large size means residual demand is lower than capacity in low consumption periods. But the extent of this modulation grew sharply last year. Lower consumption contributed to this, as did the growth of renewables. Much of France's renewable capacity is not exposed to market prices, as it is remunerated by feed-in tariffs, and has no incentive to shut down when prices fall below zero. Even the minority of capacity that is required to halt production when prices fall below zero in order to retain subsidy still can produce at prices only slightly above zero, or below the marginal cost that drives EdF to modulate down nuclear output.
Operator EdF defends its ability to modulate. The firm's nuclear chief, Etienne Dutheil, last year told a senate commission that the fleet's ability to modulate was the "envy" of other operators. And modulation has "very few effects" as long as it is partial and does not require a total shutdown that makes the plant cool down, he said. The firm told the senate enquiry that thus far, there is "no proven statistical link between modulation and a possible loss of production or increased failures of plants". But modulation could increase wear on reactors' secondary circuits and consequently increase maintenance needs, it said.
Proponents of a combined nuclear and renewables approach say that meeting France's goals for electrifying end-uses will require a large volume of extra electricity in the coming years, which potential new nuclear plants — planned for the second half of the next decade at the earliest — will not be able to deliver in time.
But opponents decry the combination of nuclear and renewables as wasteful, given that EdF does not save on any of its hefty fixed costs when modulating down nuclear plants to make way for zero-marginal cost renewable output, essentially putting a double burden on consumers that have to pay twice for two separate generating fleets.
And others put forward the damage that they say modulation does to the nuclear fleet. Rassemblement National (RN) leader Marine Le Pen raised the topic in a written question to the government last month, asserting that modulation "prematurely ages pipes and welds of reactors".
And even some internal EdF documents present modulation in a less benign light than the firm's chiefs. The combination of renewables and nuclear leads to power output fluctuations that are "never insignificant in terms of safety, especially of the control of the reactor core, and the maintainability, longevity and operating costs of our facilities", according to a 2024 report by the company's chief nuclear safety inspector.
The PPE3 energy plan, which the government hopes to finalise in law imminently, commits France to rapid increases in renewables deployment. If the plan's objectives are followed, intermittent output will grow in the coming years. The plan also aims for a rapid increase in consumption to soak up the extra power produced. But potential drivers of electrification such as heat pumps and electric vehicles had their subsidies slashed in the 2025 budget.
Increased storage capacity could be a way to integrate more intermittent renewables. France already has pumped-storage sites that can add up to 3.8GW of flexible demand during peak output periods.
But battery storage is little developed in France, thanks partly to these pumped-storage sites and to nuclear modulation, both of which limit intra-day spreads. As battery capacity grows, it typically quickly saturates the ancillary services market and these wholesale spreads will become increasingly important for making battery projects profitable.
