By Michel Rose and Marion Douet

PARIS (Reuters) - An ageing fleet of nuclear power plants and retirement of half of EDF's nuclear staff in the next 5 years are the main challenges the French nuclear safety watchdog is facing and will have to deal with, its new head said on Thursday.

France, the most nuclear-reliant nation in the world, will have to decide in the next few years whether to extend the lifespan of its 58 nuclear reactors to over 40 years, at a time it is trying to cut its reliance on the atom.

"We have a nuclear fleet which was built in the 1960s, 70s, 80s. So we arrive at a time when these plants get close to 30 years of age. The ageing of these facilities is a new factor, even if a foreseeable one," Pierre-Franck Chevet told Reuters in his first interview since he took office this month.

"Can we go over 40 years? We have no answer on that yet. We are expecting a full report from EDF which will bring us to take a decision in 2015," said Chevet, who took over from Andre-Claude Lacoste, who retired after 20 years in the post.

State-owned utility EDF, which operates all of France's reactors, has said it aims to extend their lifespan to 60 years, but there is no official limit to their functioning. Other nuclear-reliant countries have to deal with similar issues.

Japan's government, grappling with the consequences of the Fukushima disaster, has proposed to limit the lifespan of its existing reactors to 40 years.

In the United States, the 104 nuclear reactors were licensed to operate for 40 years and 73 of them have seen their licence extended by 20 years.

A campaign pledge of the new French president, Francois Hollande, was to shut the country's oldest nuclear plant at Fessenheim near the German border by the end of 2016.

Chevet said the plant could shut that year but that a definitive closure could only happen after a 5-year regulatory procedure, which had yet to start, to ensure its safety.

NEW GENERATION

Chevet also said EDF will have to deal with an ageing demographics, as the baby-boom generation born after the second world war nears the official retirement age.

EDF says about 50 percent of its nuclear personnel will have to be replaced in the next 5 years.

"We have a generation, which has worked for 35 or 40 years, which is leaving after having participated in building and starting up these nuclear units," Chevet said.

"So the underlying issue is: how do we transfer the skills in a context where the learning opportunity a reactor under construction represents isn't there anymore."

Chevet, a 51-year old engineer who started his career at the nuclear safety authority in 1986, the year of the Chernobyl disaster in Ukraine, said cooperation with European and international authorities will also be key during his 6-year mandate.

However, he warned about the risk of duplication, at a time European Union countries want to pool expertise and resources to ensure the safety of the continent's nuclear plants in the wake of the Fukushima disaster.

"Responsibilities must be clear, there cannot be several drivers in the car," Chevet said. "Once you've said that, it doesn't mean that one day, maybe for our grand-children, a single international authority won't exist, when countries and parliaments will have decided that," he added.

(Editing by James Jukwey)