* EDF says safety of reactor not affected
* Weldings designed to be unbreakable are substandard
* Regulator ASN studying technical files about the flaws (Adds technical detail, quotes, background)
PARIS, Feb 22 (Reuters) - French state-controlled power group EDF said some weldings on its new Flamanville nuclear reactor did not meet required specifications but this did not affect safety or the schedule to load it with fuel at the end of 2018.
France relies on nuclear reactors run by EDF for three-quarters of its electricity. A source familiar with the matter told Reuters that EDF would discuss the situation with the residents of Flamanville, northern France, on Thursday.
An EDF spokeswoman said 38 of 66 weldings on the Flamanville EPR reactor's secondary cooling circuit were not in line with standards aimed at making them rupture-proof.
But they did meet requirements from French nuclear regulator ASN for nuclear equipment under pressure, she said.
The regulator could not immediately be reached for comment. ASN chief Pierre-Franck Chevet has repeatedly said the start-up schedule for the reactor is tight.
The EPR reactor, the type of unit EDF is also building at Britain's Hinkley Point, is years behind schedule and billions of euros over budget.
The Flamanville reactor is scheduled to be loaded with fuel by the end of December and will ramp up throughout 2019 with a target to be at full power by the fourth quarter of next year.
EDF said in a statement on the Flamanville EPR reactor's website that deviations had been detected in the quality of the weldings in pipes transferring steam from the steam generator to the power turbine.
"EDF has started analysis aimed at proving that the mechanical characteristics of the circuit are in line with expectations, within deadlines compatible with the project planning," it said.
The problem was first identified in the workshops of third-party suppliers to Framatome, formerly called Areva, in 2015 and 2017, as well as in weldings on site.
Those steel pipes have been designed and manufactured to be 100 percent guaranteed against any rupture or leaks under EDF "high quality" standards that go beyond regulatory requirements.
EDF said those requirements had not been properly communicated to suppliers, raising questions about their quality.
EDF informed ASN about the problem in early 2017 and sent it technical dossiers to prove the equipment was fit for service and in line with regulations.
ASN ruled in June 2017 that Flamanville was fit for service despite weak spots in its steel but ordered EDF to replace the reactor cover by 2024 at the latest due to carbon concentrations in its cover, which make the steel brittle.
EDF shares were down 2.4 percent in midday trade, a steeper fall than the main Paris stock market. (Reporting by Geert De Clercq; Editing by Sudip Kar-Gupta and Edmund Blair)