EDF, France's state-owned power company, warned that several of its nuclear reactors have been affected by improper welding, sending shares of the firm sharply lower on Tuesday.
In a statement published on its website, EDF said it had been made aware of the problem by Framatome, a company owned by EDF that supplies atomic equipment.
In subsequent comments reported by Reuters, an EDF spokesman said it was too early to tell if any of its 58 reactors would need to shut down, adding that the country's nuclear regulator had been informed on Monday.
He added that Framatome had not used the same welding technique in all of the reactors it had worked in. Shares of EDF had slumped 6.8% by 1:00 p.m. London time on Tuesday.
EDF said Framatome had warned of "a deviation from technical standards governing the manufacture of nuclear-reactor components."
It added that "steam generator welds" on existing in-service components, as well as new as-yet uninstalled parts had potentially not met the standards required by the nuclear industry.
France relies heavily on nuclear power for its energy. The IAEA (International Atomic Energy Agency) said in 2018 that 71.67% of the country's electricity production is from nuclear sources — the highest percentage in the world.
Separately, a brand-new reactor at the Flamanville nuclear power plant in northern France is still under construction and has been heavily delayed after components were found to have weakened steel.
A spokesperson for EDF was not immediately available for comment when contacted by CNBC.