I published an op-ed in Lyon Mag on what the warming of the Rhône does to the power system — and on what local government can, and cannot, do about it.

The starting point: a reactor stopped thirty-five kilometres from Lyon

On 11 August 2026 at 23:59, EDF shut down reactor 3 at the Bugey plant. The Rhône was due to reach 26 °C the next day, the threshold beyond which output must be reduced or halted under environmental rules.

The plant has four 900 MW-class reactors, two of them on once-through cooling: they draw river water to cool themselves and return it warmer. In normal conditions this raises no difficulty. During a heat spike, that water degrades living conditions for fish and disturbs ecosystems — hence the limits set by France’s nuclear safety and radiation protection authority. The shutdown is therefore not a failure: it is the rule being applied, ahead of the expected breach.

The argument

• Bugey was not an isolated case. On 12 August, thirteen of France’s fifty-seven reactors were shut down or running at reduced power for environmental reasons — heat, low river flows, and even an influx of jellyfish at Gravelines. In all, 20.4% of nuclear capacity was unavailable.
• A heatwave squeezes from both sides. It cuts output at the very moment air conditioning inflates demand. According to RTE, the effect of cooling and ventilation becomes significant above 25 °C: load then rises by 0.7 to 1.1 GW per additional degree. For summer 2026 the system operator expected peaks close to 51 GW at normal temperatures, reaching 60 GW in a heatwave.
• The problem is not the volume, it is the timing. EDF puts the lost output at 0.3%. This is no collapse — it is an unavailability that lands during extreme heat, when consumption is climbing and neighbouring countries face comparable constraints. I set out this simultaneity risk in an op-ed for Le Monde.
• In the longer run, the river itself is changing. Retreating Alpine glaciers and earlier snowmelt are altering the Rhône’s summer flows. Cooling then competes directly with irrigation, navigation and drinking water. Gutting, Högselius and Burkhardt-Holm describe this tension between security of supply and environmental protection in “Atomic rivers. The (Un)sustainability of nuclear power in an age of climate change” (Energy Policy, 2025).

Who can actually act?

Responsibility is scattered, and that is the heart of the piece. The safety authority sets, for each plant, the maximum temperature of discharged water and the maximum permitted warming of the river; EDF operates the reactors; RTE keeps the grid in balance; the State decides nuclear policy and energy planning.

The City, the Lyon Metropolis and the Auvergne-Rhône-Alpes Region control neither the temperature of the Rhône nor the availability of the nuclear fleet. The regional grid-connection scheme for renewables illustrates it well: RTE draws it up with the distribution operators active in the area, and local authorities are consulted without deciding connection capacity or network reinforcement.

They are not powerless for all that. The City targets climate neutrality by 2030; the Metropolis holds levers on building retrofits, mobility, heat networks, demand management and local renewables; the Region sets the planning, energy and climate orientations. Two 2015 laws consolidated that role: the NOTRe act, which created the regional SRADDET, and the energy transition act, which handed territorial climate-air-energy plans to intercommunal authorities above 50,000 and later 20,000 inhabitants.

Hence the line I defend: the external constraint is real, but it cannot serve as an alibi. Local actors should be judged on the powers they actually hold.

A European and global problem

Hungary and Romania have had to reduce or halt output at plants cooled by the Danube. In the New York Times this summer, Ben Coates described the degradation of Europe’s great rivers and its effects on energy, industry and transport. Adapting plants, grids and river-basin management calls for a coordinated European response — necessary, but not sufficient on its own: what ultimately decides the outcome is the global fall in emissions, for which the COPs and the Paris Agreement provide the common framework.

Here is the irony: warming, driven chiefly by fossil fuels, undermines two low-carbon sources of electricity that depend on water — nuclear and hydropower. That does not condemn them. It obliges us to build future flows and temperatures into the design of new reactors and into the planning of the power system. And it makes leaving fossil fuels behind more urgent still, for the climate as much as for our water resources and our electrical resilience.

→ Read the op-ed on Lyon Mag