France nuclear output down 20% after heatwave, jellyfish

France faced an unprecedented hit to its power grid on Tuesday when 13 of the 57 reactors operated by EDF were either shut down or forced to run at reduced output, removing roughly one‑fifth of the nation’s nuclear generating capacity.

Why the outage matters

With nuclear energy supplying about 70 % of the country’s electricity, the loss of 20.4 % of capacity raises concerns over supply security, potential price spikes and the ability to meet demand during peak summer periods.

Environmental factors at play

Eight reactors were taken completely offline, while five operated at curtailed power. The majority of the shortfall stemmed from the Gravelines nuclear power plant, the largest in Western Europe, where a massive swarm of jellyfish clogged the cooling‑water pumps.

“20.4 percent of generating capacity was unavailable,” EDF said.

Scientists link the surge in jellyfish populations to overfishing, plastic pollution and warming seas, all symptoms of broader climate change. Outside Gravelines, the remaining reactors suffered from extreme heat and drought, which limited the availability of river water needed for cooling.

These conditions echo the broader Europe's latest heatwave, which has already strained energy systems across the continent.

What comes next

EDF is expected to prioritise cooling‑system upgrades and explore alternative water‑sourcing options to mitigate future heat‑related curtailments. Regulators may also review contingency plans to ensure that the grid can absorb sudden drops in nuclear output without resorting to costly imports or emergency measures.

For now, the French government is monitoring the situation closely, urging consumers to conserve electricity as the summer heat persists.