In an official statement from November 2002, the US-NRC admits:

• The NRC recognizes that nuclear power plants, including the SFP [Spent Fuel Pools],
were not specifically designed to withstand the impact of a large commercial airplane fully
loaded with fuel

• The Commission views that the nation’s efforts associated with protecting against terrorist
attacks by air should be directed toward enhancing security at airports and on airplanes

• The NRC has been in regular communication with other Federal agencies, specifically the
FAA [Federal Aviation Administration] and DOD [Department of Defense], which have
acted more than once to protect airspace above nuclear power plants

<snip>

a) None of the UK’s nuclear reactors has a containment which has been specifically designed to resist
aircraft attack other than at Sizewell B, like at the French reactors, where the secondary
containment dome is designed to resist an accidental impact of a light aircraft. In Germany and the
United States, more recent containment buildings have been designed to withstand the impact of a
military fighter jet. However, none have been specifically designed to withstand the impact of large
aircraft fully loaded with fuel.

b) None of the radioactive waste and spent fuel facilities, at the nuclear power plants, at BNFL
Sellafield and COGEMA La Hague, could withstand the directed impact of a fully loaded
commercial airliner; and

c) many of the radioactive waste and fuel storage facilities, again at the nuclear power plants and at
Sellafield and La Hague contain massive amounts of radioactive material available for suspension
and dispersal in the aftermath of a terrorist attack.

http://www.laka.org/docu/boeken/pdf/6-02-0-20-03.pdf

Many of the flights have taken place at night but the authorities have filmed and photographed some of the intrusions. Officials say they have been surprised, and alarmed, by what the images have revealed. They are not the “toy” drones that can be easily acquired and operated from a smartphone. Up to 400,000 such drones are believed to exist in France. The intruders are something much more complex and expensive – helicopter-like drones with powerful engines capable of flying for dozens of miles. They have a triangle of three white lights, a larger red light and a searchlight, possibly linked to a camera, which throws intermittent beams of light on the “target”.

Read more: http://www.independent.co.uk/news/world/europe/french-government-on-high-alert-after-unexplained-drone-flights-over-nuclear-power-stations-9850138.html