A German man obtained enriched uranium and buried it in his garden, raising concerns about the security of Germany's nuclear reactors, the environment ministry in the state of Lower Saxony said.

"How do pellets get out of a nuclear reactor? That's not supposed to happen," said ministry spokeswoman Jutte Kremer-Heye.

She said it was unclear when the man, a resident of the north-western German town of Lauenfoerder, got hold of and buried the 14 low-enriched uranium pellets, which he had sealed in a steel container wrapped in a plastic bag.

He wrote to German Chancellor Angela Merkel in December saying he wanted to hand it over, but it was not until last week that officials unearthed the pellets from his garden. Kremer-Heye said passing the letter on to the relevant government department had taken some time.

The pellets were enriched to a level of around 4 per cent, well below the weapons-grade threshold of 80-90 per cent, and tests on the area revealed no radioactive contamination. German authorities were analysing the uranium to determine its origin.

Lower Saxony's chief prosecutor Christian Gottfriedsen said unauthorised possession of nuclear fuel is a criminal offence in Germany but formal charges had not yet been filed since the investigation was still under way.