Nearly half of France's nuclear power plants are currently out of action, energy supplier Electricité de France (EDF) has confirmed.
Currently 27 of France's 56 reactors have been shut down due to routine maintenance or defects, forcing EDF to buy electricity from the European grid instead, at a time of soaring demand amid the gas crisis.
France's problems have raised questions from critics about the reliability of nuclear, and about Britain's recent big bets on the energy source.
Why reactors have been shut down
Of particular concern are five reactors that were shut down after cracking caused by corrosion was identified in pipework last year, with tests under way to determine how serious the problem is.
EDF suspects corrosion at least six more plants, and will shut down three especially for testing, and test at least another three in routine maintenance periods.
The firm, which supplies all of France's atomic energy, confirmed it was importing power from the European grid "to compensate the lack of production of our nuclear plants".
Pressed on whether it was buying more gas, the power company said it was "impossible to qualify the source of energy production".
"Of course it is a worry," said Anne-Sophie Corbeau from the Center on Global Energy Policy at Columbia University.
"This energy has to be replaced by something else, which in many cases will be fossil fuels," she told Sky News.
"Given the prices of electricity right now, this is very costly. For example, in December, we paid €1.4bn for our electricity imports, while usually we pay in the tens of millions."
A 'necessity, not a luxury', says government
The UK has just put nuclear at the heart of its new energy security strategy, aiming to build eight new reactors to generate 25% of Britain's electricity by 2050.
"The idea that nuclear reactors are always on is wrong," said Tom Burke, chairman of think tank E3G and self-described "critic" of nuclear power policy. "We just don't need [new nuclear plants]. They're very expensive."
A spokesperson for Britain's energy department called nuclear power "a necessity, not a luxury" and the "only form of reliable, low carbon electricity generation which has been proven at scale".
The UK government is also backing wind, solar and North Sea oil and gas in a bid to reduce dependence on foreign gas, and investing in ways to reduce energy demand.
Climate campaigners have criticised plans for the North Sea, the lack of onshore wind and the cost of nuclear.
'High level of safety', says EDF
EDF told Sky News the corrosion in five of the offline reactors "does not call into question the high level of safety of our facilities".
"We propose to implement the principle of safety primacy on the technical subject of stress corrosion, which we are currently encountering," a spokesperson said.
The remaining 22 plants are down either for routine yearly maintenance, taking about five weeks, or for 10 or 40 year safety reviews, which take longer, and due to delays from the pandemic.
The setbacks forced EDF to lower its expected 2023 output by about 40 terawatt-hours (TWh), with reduced availability expected well into next year. In February global credit ratings agency S&P downgraded EDF's rating, forecasting its earnings to fall by €5-7bn, due to the reduced output as well as regulatory measures from the French government to limit price increases.
France built most of its reactors at pace in the late '70s and '80s - after the 1970s oil crisis - expecting to use them for 40 years. Now EDF is seeking to extend their lifespan by ensuring they are just as safe as new reactors, but that requires time and money.
Nuclear usually generates about 70% of France's electricity, explained Ms Corbeau. At 2pm on Thursday afternoon, France's National Grid RTE had the figure at around 58%.
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