(Bloomberg) -- Electricite de France SA extended maintenance halts at two nuclear reactors by four months and warned it may have to carry out lengthy repairs at seven others next year, further straining European power supplies.

The world’s largest nuclear fleet has suffered prolonged shutdowns this year that have left Europe even more reliant on natural gas at a time of record prices. While EDF has returned many units to service in recent weeks, its aging plants look set to run far below optimal capacity this winter. That could worsen an energy supply crisis that’s already cost Europe $1 trillion to try to contain.

The restart of EDF’s Penly-2 unit has been delayed to June 11 from Jan. 29, while its Golfech-1 generator will also be back online June 11 rather than Feb. 18, the utility said Monday in a message to the grid operator.

In addition, the halt of Cattenom-3 is extended by one month to March 26, and the restart of Civaux-2 is postponed by more than a month to Feb. 19.

Read: French Nuclear Reactor Availability

France has been forced to import power, having traditionally been a major exporter to neighboring countries. Its reactors are now running at just over two-thirds of typical capacity. That means more electricity will have to come from gas, eating into European storage that’s become more difficult to refill after Russia cut supplies.

France’s grid operator has flagged the risk of a potential power shortfall in the colder months as heating demand rises while EDF continues to grapple with reactor repairs.

Read: France Is Losing Its Race to Fix the Reactors Europe Needs

Multiple plant shutdowns have resulted from stress corrosion cracks on pipes in the reactors’ cooling systems. Since EDF uncovered the problem at the end of 2021, it has found that 16 of its 56 reactors are more prone to the issue than its older units because of their designs. Ten of these have been fixed this year or are still undergoing repairs.

On Friday, the utility said it’s now considering fully replacing the cooling-system pipes on the remaining six “sensitive” reactors as a preventative measure. It may also opt for a full pipe replacement at Cattenom-1, which is among units already halted for repairs.

One country affected by France’s nuclear troubles is the UK, which is linked to mainland Europe by giant cables and has typically been able to tap French supply when needed. The outages put Britain in a more perilous position, especially when temperatures sink and calm weather crimps wind energy, as happened last week. In that instance, the UK was able to import sufficient power from Europe, but France’s nuclear woes increase the risk of a shortfall.

And it’s not only EDF’s existing plants that have problems. In a further setback to France’s nuclear program, the company said last week that it’s delayed the startup of a new reactor in Normandy by several months into 2024 due to extended work. That project is already more than a decade late.

--With assistance from Francois de Beaupuy.

(Updates with EDF warning of future repairs starting in first paragraph.)

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