France ran the grid in reverse

On June 23, 2026, France logged its hottest June day since records began in 1947, over 44°C. The problem for its nuclear fleet was downstream: reactors on the Seine, Rhône and Garonne draw river water for cooling and return it warmer, and French rules cap that discharge at roughly 28°C to keep the rivers survivable for fish. As the rivers themselves crept toward that ceiling, EDF had almost no headroom left to add heat, and throttled or shut reactors at Nogent, Saint-Alban, Bugey and Golfech.

The relevant planning quantity is no longer France's annual surplus but the capacity it can deliver under thermal stress.

The visible effect wasn't a blackout — it was a role reversal. France normally floods its neighbours with cheap nuclear power; on the peak afternoon its export surplus fell from a usual 11–12 GW to under 3 GW, and for a few hours the continent's biggest electricity exporter ran in reverse, pulling in cheaper Spanish solar. Over the full month exports stayed ordinary and the grid operator said supply was never at risk, so this was a squeeze on the surplus, not a shortage.

It is also not new, which is the actual signal. France curtailed river-cooled reactors in 2003, 2018, 2019, 2022 and — more severely — 2025, when over 7 GW went offline. What used to read as a freak event now recurs on a schedule tied to the calendar, and it bites hardest on the hottest afternoons, exactly when demand across the interconnected European grid peaks too. The number worth watching is no longer France's comfortable annual surplus but how much it can actually deliver when the rivers run warm.