French safety officials on Thursday gave the green light to extend the lifetime of the country's oldest nuclear power plants as it seeks to boost the share of renewables in its power mix.

Nuclear energy currently provides nearly 70 percent of French electricity, more than in any other country.

France, hoping to reduce that share to 50 percent by 2035 -- a target pushed back from an earlier 2025 date -- with the help of renewables, has been holding off from building new reactors.

The number of French reactors, at 56, is second in the world only to the United States which operates 85.

France's oldest nuclear reactors are getting another lifeline
France's oldest nuclear reactors are getting another lifeline AFP / JEFF PACHOUD

The French nuclear safety authority (ASN) said the country's 32 plants with 900 Megawatt capacity, built mostly in the 1980s, would be allowed to operate for another decade, taking their potential lifespan to 50 years from the initially planned 40.

They will therefore not be decommissioned before the late 2020s or even late 2030s, depending on their initial launch date.

The safety of French nuclear plants is checked every decade.

ASN asked state-controlled electricity provider EDF, which manages the country's nuclear plants, to undertake any necessary work to safeguard their security.

The main target was to "limit the consequences of any accident, especially any serious accident involving the meltdown of a reactor", ASN's deputy director-general Julien Collet told AFP.

France checks its nuclear reactors every 10 years
France checks its nuclear reactors every 10 years AFP / GUILLAUME SOUVANT

Another objective was to improve the resistance of the plants to outside shocks including earthquakes, floods, extremely hot weather, or a fire in the reactor.

Anti-nuclear campaigners have long demanded the closure of veteran nuclear power stations, and last year obtained the decommissoning of France's oldest plant at Fessenheim in the east of the country.

"Active French nuclear power plants were built to operate for 30 or 40 years. Beyond that, nuclear reactors enter an unknown ageing phase," said NGO Greenpeace, calling for more plants to be closed.

ASN president Bernard Doroszczuk told the Ouest France newspaper that there were still "weak points" in the stations' security equipment, requiring "vigilance", but that there had been improvements.

France's nuclear reactors, grouped in 18 sites, are all second-generation pressurised water reactors.

EDF in 2015 estimated the cost of dismantling all the reactors at 75 billion euros ($92 billion) but a parliamentary report said the real cost would be more.

A third-generation reactor called EPR and under construction since 2007 in Flamanville in northern France was supposed to go online in 2012, but the launch date has been delayed repeatedly and is now fixed for next year.

Flamanville's cost has run over 10 billion euros, more than three times the initial estimate.

Once operational, it will have an estimated life span of 60 years.