French nuclear fleet hits record 20.4% unavailability as heat, drought and jellyfish strike 13 reactors

Record environmental unavailability

On 12 August, shortly before 10:00, EDF's nuclear reactor fleet reached a new record of unavailability for "environmental constraints" or "external causes related to the environment," according to AFP calculations based on EDF data published since 2015. The rate of missing power reached 20.4%, with 13 of the fleet's 57 reactors affected by either shutdowns or production reductions. The causes span drought, high temperatures, and a massive jellyfish influx at the Gravelines plant in the Nord department of northern France. The figure marks a new peak both in the number of reactors affected and in missing power output, according to the AFP analysis. The data series used for the calculation has been published by EDF since 2015, making this the highest environmental unavailability recorded in that period.

Jellyfish overwhelm Gravelines

At the Gravelines nuclear plant, located between Calais and Dunkirk, a massive influx of jellyfish on 10 August obstructed the seawater pumping stations used to cool the reactors. EDF announced the shutdowns on 11 August. Production units 3 and 4 shut down automatically, while unit 2 was shut down manually by EDF teams. A fourth reactor's output was halved. An EDF spokesperson described the scale of the intrusion to AFP.

Several tons of jellyfish entered the seawater pumping stations and clogged the filter drums.

With one of the plant's six 900 MWe pressurized water reactors already in maintenance, only one reactor at Gravelines was running at full capacity as of 12 August. The Gravelines plant, which draws North Sea water for cooling, is the largest nuclear facility in Western Europe. The six reactors at the site each have a capacity of 900 MWe.

Safety assurances and protocol

EDF stated there were no consequences for the safety of the installations, personnel security, or the environment. The plant activated a security protocol established after a similar jellyfish episode exactly one year earlier, The Times reported. That previous incident had forced the plant into a 10-day shutdown after four reactors automatically shut down. Following the earlier episode, EDF introduced new monitoring and dredging techniques and launched a partnership with a fishermen's association, according to Reuters. The Times noted concerns that a massive swarm of jellyfish could obstruct the water cooling systems of the plant, which sits between Calais and Dunkirk. After the previous incident, the reactors resumed operation after a few days.

Summer of strain on French nuclear

The jellyfish episode compounds a summer of environmental pressure on France's nuclear fleet. Between June and July, EDF had to halt several reactors due to high temperatures, while heatwaves and drought complicated operations across the fleet. The 20.4% unavailability figure, calculated by AFP from EDF data published since 2015, represents a new record for environmental unavailability. At Gravelines alone, three full shutdowns and one production reduction account for a significant share of the 13 reactors affected fleet-wide, with the remainder linked to heat and drought conditions at other plants across France. The heatwave currently affecting France has put nuclear production under sustained pressure throughout the summer months.