A spokesman for EDF, the energy firm that runs the country's nuclear plants, said that a lightweight motorised aircraft with one person inside got into the plant, while another person guided the craft outside the facility.
In December, Greenpeace activists broke into two nuclear power plants in France in an action the group said was aimed at exposing the vulnerability of the facilities.
Nine activists broke into the Nogent-sur-Seine plant near Paris and two into the Cruas plant in southern France.