The Gelatinous Gridlock: How a Jellyfish Swarm Shut Down a Nuclear Power Plant

In northern France, a sudden swarm of jellyfish has paralyzed operations at one of the country’s largest nuclear power plants—Gravelines. On Sunday evening, a massive influx of these marine creatures clogged the cooling water intake systems, triggering the automatic shutdown of three reactors. The fourth was halted shortly afterward, leaving the entire complex—supplying electricity to roughly five million homes—completely offline. The remaining two reactors were already out of service for scheduled summer maintenance.

Gravelines draws its cooling water from a canal connected to the North Sea, where several jellyfish species proliferate during the warmer months. When these swarms reach the coast, they can quickly overwhelm the intake filters designed to block large debris and marine life. If the flow of water to the cooling system is obstructed, automated safety systems immediately shut down the reactors to prevent overheating.

France’s state-owned energy company EDF described the invasion as “large-scale and unpredictable,” emphasizing that it posed no risk to staff, facilities, or the environment. Electricity exports to the United Kingdom remained unaffected. Nevertheless, the very nature of such incidents presents a serious challenge for coastal power stations that depend on a constant, abundant supply of cold seawater.

Mass jellyfish blooms are a natural phenomenon, often occurring when water temperatures rise and ocean currents shift. Though harmless to humans at a distance, they can cause significant disruptions to both nuclear and thermal power plants. Similar events have been recorded worldwide: in 2021, jellyfish clogged the filters of Scotland’s Torness nuclear power station, forcing a shutdown—an incident that also occurred there in 2011. In Sweden, the United States, and Japan, “jellyfish blockages” have temporarily halted operations at both nuclear and coal-fired plants.

Some cases have been particularly severe. In 1999, a colossal bloom in the Philippines disabled a power plant, triggering a major blackout initially mistaken for a Y2K glitch or even an attempted coup. More recently, in September last year, staff at eastern China’s largest coal-fired power plant battled for ten days to clear more than 150 tonnes of jellyfish from its cooling system.

Scientists increasingly link the rise in such events to climate change, overfishing, and the restructuring of marine ecosystems. The decline of natural jellyfish predators, such as sea turtles, combined with environmental changes, creates conditions for more frequent and larger blooms, which then drift toward shore.

Researchers at the University of Bristol are developing an early-warning system to forecast mass jellyfish blooms and reduce the risk of power plant shutdowns. For now, Gravelines’ reactors will remain offline until the intake systems are completely cleared. Given that the North Sea’s warm summer waters persist, engineers and marine biologists will be monitoring conditions closely, wary of the next unpredictable wave of these gelatinous disruptors of the energy grid.