The automatic shutdowns of four units "had no impact on the safety of the facilities, the safety of personnel, or the environment", EDF said on its website.

"These shutdowns are the result of the massive and unpredictable presence of jellyfish in the filter drums of the pumping stations," the operator of the Gravelines nuclear plant said.

The plant is fully shut after the incident, with its two other units already offline for maintenance.

Three of the production units automatically shut down late Sunday evening, followed by a fourth early Monday morning, said EDF.

"The plant's teams are mobilised and are currently carrying out the necessary diagnostics and interventions to restart the production units in complete safety," EDF said.

Gravelines is Western Europe's largest nuclear power plant with six reactors, each with the capacity to produce 900 megawatts.

The plant is due to open two next-generation reactors, each with a capacity of 1,600 megawatts, by 2040.