Environmental group Greenpeace said yesterday it had exposed the “vulnerability” of French nuclear sites after its activists broke into an atomic power station near Paris before being arrested.

French authorities admitted to security “lapses” after the incident and vowed a full investigation, while President Nicolas Sarkozy denounced the activists as “irresponsible”.

The dawn raid saw nine activists sneak past security at the Nogent-sur-Seine plant, 95 kilometres southeast of Paris. Most were quickly arrested but two managed to evade capture at the plant for nearly two hours, authorities said.

“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 campaigner.

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

French authorities confirmed the intrusion, saying that the nine had been detained and that despite Greenpeace’s claims to the contrary there were no signs that any more had managed to infiltrate other installations.

Greenpeace was saying late yesterday afternoon that it still had activists hiding in at least one other nuclear site.

“There are no traces of intrusion in EDF’s other nuclear stations,” said the French energy giant which runs the nuclear plants that France relies on for 75 per cent of its energy,

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

“We saw immediately that they were not armed ... As soon as these people entered they were detected and followed, and when you are dealing with people with peaceful intentions, you must intervene calmly,” Dominique Minière, the head of EDF’s nuclear programme, told a news conference.