Three reactors at France's Gravelines nuclear power plant were shut down late on Monday due to a massive swarm of jellyfish, triggering preventative measures put in place last year after the entire plant went offline, operator EDF said.
The plant in northern France is one of the largest in the country and is cooled from a canal connected to the North Sea. Its six units produce 900 megawatts of power each, or 5.4 gigawatts in total.
Reactors 2, 3 and 4 were taken offline due to the swarm and reactor 1 was limited to 50% of output as a preventative measure. Reactor 5 was already offline for maintenance and reactor 6 remains fully operational, EDF said.
The complete outage last year prompted the operator to implement new monitoring and trawling techniques to try to prevent or limit the impact of future events.
EDF has installed fixed cameras at the intake canals and on the filter drum screens to detect any incoming blockages as soon as possible, an engineer told Reuters.
The company has also partnered with a French fishing organisation and a sea rescue organisation, which monitor for swarms and can raise an alert to launch targeted trawling operations by fishermen to reduce the swarm's density before it reaches the plant, the engineer said.
The fishing boats were deployed on Saturday after an initial alarm was raised, limiting the impact from the swarm. The jellyfish that are already on site are being removed with vacuum trucks, he said.
No new jellyfish are arriving, so cleanup is finishing, which should lead to a quick restart of the units, they added.
The situation does not have any impact on the safety of the facility, the personnel or the environment, EDF said.
Scientists warn such events could become more common due to factors including global warming, the arrival of invasive species, habitat loss of predators and over-fishing.
EDF is also grappling with heat-limited nuclear reactors across the country, as a fifth heatwave is expected to grow throughout the week and curtail French nuclear output.
One reactor in the north has been offline for most of the summer due to low river levels, while another reactor has had limited restarts and remained largely offline for most of July.
