Greenpeace specifically examined the outer shells of Germany's 17 nuclear power plants and found those of eight older reactors to be highly vulnerable to an attack.
The walls of containment buildings at, for example, the Unterweser, Kruemmel and Neckarwestheim nuclear power plants were about one meter thick, said Oda Becker, a physicist at Hanover University and author of the study.
That was too thin to protect even against a hit with "a conventional anti-tank missile using modern thermobaric warheads."
The Russian-built AT-14 was one of the most frequently used weapons of this kind, she said and added "just one of these missiles isn't enough, but fewer than ten shots could trigger a horrible scenario."
Massive radiation damage
In a likely scenario drawn up by Becker, a warhead penetrating the outer shell would cause a massive explosion, destroying most of the plant's interior, including vital utility and security systems.
This would be followed by "a meltdown of the reactor's core" and the release of radiation in an "open-containment situation," meaning that radioactivity is released into the environment.
"Within just a few hours, radiation will create a 25-kilometer zone of death around the plant," she said, adding that one third of Germany "would have to be evacuated."
Becker based her study on tests made in Russia recently, which suggest that reactor containment buildings need to have walls at least 3 meters thick to sustain a hit with an AT-14 missile.
Russia had carried out the tests because it was much more threatened by terrorism, Becker said, and because it was aware of the potential damage its AT-14 weapon system could cause.
After all, Russia had exported AT-14s in large numbers, including to countries suspected of harboring terrorists "like Yemen, Syria and Algeria."
'Shut them down immediately'
Greenpeace dismissed allegations of scaremongering in connection with its study.
"But the German government must take our findings seriously, and immediately shut down the older, most vulnerable nuclear power plants," said Heinz Smital, a nuclear expert with Greenpeace Germany.
Smital also criticized the government's recent decision to prolong the lifetimes of German reactors by an average of 12 years, saying the move would create an "incalculable security risk" for the German population.
In an accord with Germany's nuclear operators, Berlin decided against insisting that reactors be upgraded to protect against a terrorist attack with an airplane.
The government apparently considers the risk of such an attack to be limited and the costs too high, at around a billion euros per plant.
Greenpeace says it intends to use its report as the basis for bringing a legal case against the government, and argues that Berlin is grossly neglecting its duty to ensure the right of citizens to a healthy and secure life.
Author: Uwe Hessler
Editor: Susan Houlton