The European Pressurised Reactor (EPR) in Flamanville, Normandy is going through a "long and complex startup process requiring many trials and tests, and that can induce shutdowns like this," a spokeswoman for state-owned energy giant EDF told AFP.

The shutdown "proves the safety system is working well," she added, saying that staff were now "doing the necessary technical checks and analysis... then they will restart the reactor".

EDF's latest reactor, supposed to be the model for a new generation of power plants pushed by President Emmanuel Macron for the coming decades, has been completed 12 years late at a cost of 13.2 billion euros ($14.6 billion) -- around four times the 3.3 billion initially budgeted.

Reactors of the same design have previously been completed in China and Finland.

"On the Finnish EPR, there were several setbacks, especially with some hydraulic pumps that were faulty and had to be replaced," said Nicolas Goldberg, an energy expert at Colombus Consulting.

"This doesn't call the startup into question. We'll just have to be patient," he added.

EDF had on Monday secured approval to begin the fission reaction from France's ASN nuclear safety authority, having loaded uranium fuel into the reactor in May and carried out a battery of tests.

The Flamanville plant will gradually ramp up to 25 percent output before being connected to the grid "by the end of autumn", the power company says -- a further delay from its previous target of the end of summer.

At full power it will be France's top-producing nuclear reactor at 1,600 megawatts, enough to supply around three million homes with electricity.