This story goes back to October 5, when French energy company Electricite de France began seeing things in the sky above three of its nuclear reactors in the southwest of the country.

They saw drones flying over the facilities - over several areas at the same time. But the company had no idea who was operating the drones or why.

Since then, more than 30 drones have been spotted.

Oda Becker is a physicist and nuclear consultant, who has written a report on the drones.

DW: What kinds of drones are we talking about?

Oda Becker: Mid-sized. Larger than a remote-controlled, multi-copter. Something like a drone that could deliver medicine or packages, like those used by parcel services.

So Greenpeace is worried that terrorists might use these drones to do what, exactly?

Insiders could use the drones to deliver explosives, and then trigger an accident. They could prepare an attack from the air - figure out whether there are any security systems to stop an attack with a helicopter, for example. And they could spy on the ground - the security, all the details - so that they can attack the facility [from the ground] with an anti-tank weapon.

If I were a terrorist, which area of a nuclear facility would I want to strike to do the most damage?

There are two targets that could result in a very major release [of nuclear material]. One is the reactor building itself. The reactor core is only protected by a wall measuring 90 centimeters (three feet). [Newer reactors - built today - have two-meter walls - the ed.] So it could be destroyed by various devices.

And the other point is - and this is very particular to France - the spent fuel rod buildings. They are only protected by a very thin wall - 30 centimeters - and just a very thin metal roof. And there could be more nuclear materials in the spent rod building than in the reactor itself.

No protection

So a thin metal roof is the only protection between us and a room full of nuclear materials.

Yes.

And the specific threat is that an explosive device could be detonated in that room?

The threat is that the water from the spent fuel pools would drain. If that happens, there will be a meltdown of the fuel, and a release of a big amount of radioactive material.

But how could they drain a pool with just one drone? Isn't that difficult?

Terrorist groups have the capabilities to figure out a scenario.

Let's go back to the thicker wall protecting the reactor building. Could a drone really break through a cement wall that's almost one meter thick?

You can't destroy such a wall with a drone. You'd need nearly 100 kilograms [220 pounds] of explosives. But there are other possibilities. I think more credible is a scenario where a terrorist group uses the drones to spy, and then carries out an attack with a helicopter, for example.

And if you [break that wall], you also destroy the cooling system of the reactor core. And without the cooling, there's a meltdown - and a very quick release in the environment.

The threat spreads

What would the consequences be for France and Europe?

It could spread up to 300 kilometers (180 miles).

So by that math, the nuclear materials wouldn't reach Germany, since the plants are in southwestern France?

Of course they would reach Germany. Near the border, the areas would have to be evacuated, and people wouldn't be able to come back for decades. But the important point is the speed of the nuclear release. It would not be possible to evacuate the people as fast as necessary. And that's exactly the danger.

Depending on the weather, it could spread to Italy, Sweden, Spain, Greece. But obviously the most threatened areas are in Luxembourg, Germany, and Belgium - and of course, France itself.

Are the risks limited to the three, older nuclear plants in southwest France?

There are 34 in the 900-megawatt class in France. And they are all as badly protected. The spent fuel rod facilities, especially.

If I'm France's energy minister, I'd probably say, 'Well, we need the electricity - we're not shutting them down.' So what can France do to protect itself in the meantime?

To be honest, I have no idea. Because they've tried since the beginning of October. But they haven't stopped the drones.

Air traffic control doesn't pick up civilian drones below a certain size or flight ceiling, but couldn't the French military use more powerful radar to figure out where these drones are taking off and landing?

I think in China they have devices to deal with civilian drones. France doesn't have any.

So how do you protect the facilities?

Make thicker walls.

Oda Becker is a scientific consultant on nuclear safety based in Hanover, Germany. Her clients include the Austrian federal government as well as several non-governmental organizations (NGOs). Her studies include the European "Stress Tests" in the aftermath of the Fukushima accident and analyses of accident scenarios from flooding or terrorist attacks on nuclear facilities (for example, the crash of a large commercial airliner or the impact of an anti-tank weapon).