Jellyfish swarm forces shutdown of three reactors at France's largest nuclear plant

EDF took three reactors offline at its Gravelines nuclear power plant on Monday after swarms of jellyfish clogged the seawater intake channels used for cooling. A fourth unit had its output halved, leaving only one of the site's six 900-megawatt reactors running at full capacity. A sixth unit was already shut for planned maintenance.

The operator said the shutdowns pose no risk to safety, personnel or the environment. The plant, located near Dunkirk, is the largest nuclear facility in Western Europe and supplies a significant slice of France's baseload electricity.

Mounting pressure on French nuclear fleet

France's nuclear availability has been under strain for months. Extreme heat and drought forced EDF to curtail output at several plants in June and July, and AFP calculations based on EDF data show nuclear generation was already running 15.6 percent below normal as of Sunday. The latest losses at Gravelines remove roughly 2,700 megawatts of firm capacity at a time when European gas markets remain tight.

A recurring problem

This is not the first time jellyfish have disrupted the site. A similar bloom forced a temporary shutdown a year ago. Comparable events have hit reactors in Sweden in 2013 and Japan in 1999. Marine biologists attribute the rising frequency to overfishing, plastic pollution and warming oceans, conditions that favour jellyfish over their natural predators.

This situation has no impact on the safety of the facilities, the safety of personnel, or the environment.

EDF has not given a timeline for restarting the affected units. The utility will need to clear the intake screens and verify that cooling flows are stable before returning the reactors to service. With another heatwave possible in August, grid operators will be watching the plant's availability closely.