Nuclear energy has been a source of protests this year Germany's Environment Minister, Juergen Trittin, has demanded clear answers to how a worker apparently managed to steal plutonium from a nuclear reprocessing plant.
He said he wanted a "watertight explanation" from regional authorities over what he described as "scandalous security failures".
Police found a package containing liquid plutonium buried on a disused military airbase at Landau, near the French border.
A 49-year-old worker from the reprocessing plant near Karlsruhe, who was reportedly taken into custody on Thursday, has been tested showing radiation levels several hundred times above the accepted limits.
He reportedly led the police to the site at Landau.
The plutonium was packaged in a 5cm tube and wrapped in latex gloves. Some of the liquid had apparently seeped into one of the gloves.
The worker denies stealing the plutonium and claims he took the material by mistake.
His partner and child, who also tested with high levels of radioactivity, have been admitted to hospital and his flat has been sealed off.
Environmentalists are taking legal action against the plant for negligence and a lapse in nuclear security.
The plant, which was shut down in 1990 and began to be decommissioned in 1996, will also be checked by Euratom inspectors.
The regional environment ministry has dismissed suggestions that the worker may have been trying to smuggle the plutonium for use in nuclear weapons.
A spokeswoman said that the plutonium was not weapons-grade and could not be turned into weapons-grade material.
Nuclear energy has been a hot issue in Germany this year, with the signing of an agreement to phase out the use of nuclear energy over the next 20 years.
There have also been wide-scale protests by environmentalists over the transportation of nuclear waste to France and Britain.
Transportation of the waste was resumed this year after a three-year ban due to safety fears.