The activists climbed on top of a reactor building and unfurled a banner, said a Greenpeace spokesman. The power company, Electricite de France (EDF), says the intruders were detected straight away.
Seven out of nine activists who entered the site have been arrested, said the firm. Greenpeace also targeted two other nuclear sites in France at the same time.
Banners were unfurled at those sites, say police, but it is not clear whether the activists managed to gain entry.
The campaign group says it did succeed in putting up a banner on the Nogent-sur-Seine plant which read “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 nuclear reactor,” said a Greenpeace nuclear specialist, Sophia Majnoni.
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,” said an EDF statement.
This does make one think about the security of access to nuclear power plants” Henri GuainoAdvisor to President Sarkozy
French Industry Minister Eric Besson expressed surprise when told of reports about the Greenpeace action.
“That would mean there has been a dysfunction and that measures must be taken to ensure that it doesn’t happen again,”he told French radio.
An advisor to President Nicolas Sarkozy, Henri Guaino, said the Greenpeace action was “irresponsible”, but acknowledged that it raised concerns.
“Conclusions must be drawn from this…This does make one think about the security of access to nuclear power plants,”French news agency AFP reported him as saying.