According to the police, some 30 people have “illegally gained access to the outside security area” by climbing up the 60-metre-high structure.
Greenpeace said the daylong protest was meant to highlight the “deadly danger” of nuclear power and demanded the immediate closure of the older reactor and six other facilities in Germany.
The group said none of these plants was sufficiently secured against a potential plane crash or terrorist attack.
“Those politically responsible are hiding and playing down the danger to people for the huge profits of nuclear firms,” said Greenpeace's nuclear expert Heinz Smital in a statement.
Germany has agreed to phase-out nuclear power, however, some conservative politicians have mooted the possibility of reversing or slowing this decision to help the country meet its CO2 reduction and energy production goals.