EDF said that it would use robots to carry out the tricky repairs on the plant it has been building for over a decade on the country's north coast.

France's nuclear safety authority ASN ordered the work.

The welds would take the cost of the novel European Pressurised Reactor (EPR) reactor from 10.9 billion euros -- already three times the initial estimate -- to 12.4 billion euros, it added in a statement.

EDF shares fell 1.9 percent to 9.32 euros in mid-afternoon trading on the Paris stock exchange before rallying slightly.

The electricity giant had studied three ways of repairing eight faulty welds on the reactor's containment building that were flagged up by the ASN in June.

It said it had decided robots were the best solution for the "high precision operations" but must now wait for the plan to be approved by the ASN.

When EDF began work on the reactor in 2007 it was targeting a launch date of 2012.

But the project, which was meant to showcase the third-generation EPR reactor technology that EDF has sold to Britain and Finland, has been dogged by problems.

Like the plant's cost, the date for its completion has been repeatedly revised.

After an initial delay to the end of 2019, EDF confirmed Wednesday it was now only aiming to load fuel into the reactor at the end of 2022.

The announcement of the new overruns at Flamanville come two weeks after EDF warned that the controversial EPR reactor it is building on Britain's south coast also faced further budget overruns and delays.

The company said the Hinkley Point plant would now cost between 21.5 and 22.5 billion pounds (24.4-25.5 billion euros/$26.8-28.1 billion), a rise of between 1.9 and 2.9 billion pounds over previous estimates.

EDF has blamed the problems with that plant largely on difficult ground excavation conditions which required adaptations to meet British regulations.

- 'Hard times' -

The French government reacted with exasperation to Wednesday's announcement, with the environment ministry saying it was "not satisfied with this situation and awaits explanations".

It said that an audit it commissioned into the Flamanville project, due to be submitted by the end of the month, would "shed full light on the causes of the delays and cost overruns".

In early October, EDF CEO Jean-Bernard Levy admitted that the French nuclear industry was going through a "hard time" and promised to "draw the lessons from this situation".

A few months ago EDF's board discussed abandoning the Flamanville project but the French state, which has a majority stake in the company, still supports the build.

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EDF - ELECTRICITE DE FRANCE