Paris: Greenpeace on Monday said it had exposed the "vulnerability" of French nuclear sites after its activists broke into and reached the heart of an atomic plant near Paris before being arrested.

The dawn raid saw nine activists sneak past security at the Nogent-sur-Seine plant, 95 kilometres southeast of Paris, and unfurl a banner on one of its domes reading: "Safe Nuclear Power Doesn't Exist."

"The aim is to show the vulnerability of French nuclear installations, and how easy it is to get to the heart of a reactor," said Sophia Majnoni, a Greenpeace nuclear expert.

"In about 15 minutes the activists reached the heart of the plant, where the nuclear core and nuclear fuel are," she told journalists near the plant.

French authorities confirmed the intrusion, said that the nine had been detained and a thorough search was being conducted of all installations after Greenpeace said its activists had entered other sites and remained present.

"We at the moment have peopled who are in a nuclear site and have not been located," Greenpeace spokeswoman Adelaide Colin said.

French energy firm EDF, which runs the nuclear plants that France relies on for 75 per cent of its energy, said there were no signs that activists had managed to infiltrate other installations.

"There are no traces of intrusion in EDF are other nuclear stations," it added.

EDF sought to downplay the incident, insisting it had been aware of the intrusion from the start.

The activists "were immediately detected by the security system and were permanently followed on the site, without a decision being made to make use of force," the company said in a statement.

Authorities insisted the incident had posed no risk.

"At no point was there a threat to the integrity of nuclear installations," interior ministry spokesman Pierre-Henry Brandet said.

"We are proceeding with an extensive search of all nuclear plants and installations," he said.

Authorities said attempts had also been made to break into nuclear power plants in Blayais in southwestern France and
Chinon in central France, as well as a nuclear research centre in Cadarache in the southeast.